A total number of 1200 naturally infected fishes (600 Oreochromis niloticus and 600 Clarias gariepinus), were subjected to clinical, post-mortem and bacteriological examinations. The bacteriological examination revealed the presence of 268 Streptococcus species isolates. The prevalence was 22.33 % among the examined fishes.
Streptococcal infection for O. niloticus and C. garepinus was highest in summer and lowest in winter. Streptococcus spp. infection rate for O. niloticus was highest in Kafr El-Sheikh and lowest in Sharkia, while, for C. gariepinus was highest in Kafr El-Sheikh and lowest Domyata governorates in Egypt.
Bacteriological examination revealed the isolation of 268 different bacterial isolates. Isolates were isolated from kidney, liver, ascetic fluid, intestine and spleen. The bacterial isolates were identified as 5 species Streptococcus faecium (42.9%), Streptococcus faecalis (29.48%), Streptococcus sp1. (11.57%), Streptococcus sp2 (8.96%) and Streptococcus sp3 (7.09%). Antibacterial sensitivity testes revealed that there were differences between strains in the sensitivity against different antibacterial discs. Experimental injection of fish revealed that there is no difference in susceptibility between normal tilapia and monosex tilapia.
Analysis of proteins of the isolated Streptococcus spp. revealed presence of common characteristics bands (18 and 35KD) which characteristics to Streptococcus spp. and Enterococcus faecalis. The tree for RAPD-PCR that resulted from cluster analysis noticed that there were two main groups (standard Streptococcus faecalis and Streptococcus faecalis) as one group, and Streptococcus sp.2 and Streptococcus sp.3 as a second group, while the Streptococcus faecium branched to one subgroup from the Streptococcus sp.2 and Streptococcus sp3 group. But the Streptococcus sp.1 presented as out-group.
RAPD-PCR analysis revealed that seven primer out of ten primers yielded number of polymorphic and share bands and three primers failed to give shared bands. 65 DNA bands were detected, 11 shard bands.

The growth performance, reproductive and survival rate for different crosses of Nile tilapia (Oreochromis niloticus) collected from four different locations (Abbassa, Manzalla, Maryout and Aswan) were evaluated. At the end of growing period in hapa (90 day) at summer, showed that the male Manzalla and female Maryout were the highest weight gain (39.83 g. and 41.25 g. respectively), while male Aswan and female Manzalla were the lowest weight gain (11.68 g. & 15.49 g. respectively). The growth rate of the sixteen crosses populations obtained from first generation (F1) through winter season in earthen pond (120 day) and summer season in hapa (60 day) were evaluated. The cross of Abbassa x Maryout showed the highest weight gain at the two seasons 48.0 g at winter and 80.4 g at summer, while the cross of Manzalla x Abbassa showed the lowest weight gain at both seasons.
The survival rate (SR) to 16 different crossing populations through winter season (120 days) in hapa suspended in earthen pond was low. On the other hand, the crossing between males Aswan and female Manzalla, Manzalla x Manzalla and Abbassa x Maryout were (79.37%, 75.00% and 56.06 respectively) higher than populations of Abbassa x Abbassa (16%). While, survival rate through summer season (60 day) in hapa showed that highest in crosses populations Aswan x Manzalla, Maryout x Aswan, Abbassa x Manzalla and Abbassa x Maryout (64.5, 60.0, 57.9 and 50.0% respectively) and very low in population of Abbassa x Abbassa (5.7%). The relative fecundity was highest in Aswan population; female gave 3.62 fries per g. body weight, while, Maryout population was the lowest one; female gave 2.29 fries per g. body weight. That is to say, selective breeding program can be established as method in aquaculture for genetic improvement of fishes with regard to better growth rate, survival rate, relative fecundity and adverse environmental factors especially to variations in the temperatures exposed to it

The Nile tilapia (Oreochromis niloticus) were collected from six locations in Egypt, Abbassa (Ab) Sharkia, Assute (Asu), Kafer El-Sheikh (Kf) (Lake Brollus), Lake Manzala (Mz), El-Menia (Mn) and El-Qanater El-Khiria (Q). Ten individuals from each population were sampled and dissected. The results revealed that Assute population had the higher similarity with an average (88%) followed by Qanater (87%), El-Menia (86%), Abbassa (84%) and Kafr El-Sheikh (83%) while the Manzala represented the lowest similarity with an average (79%). All population had sex-specific bands in two sexes with the exception of El-Menia population that had only female-specific band and “Q” population that had only male-specific band. Variations among locations herein may be due to spatial isolation across the Nile river and its branches. The diversity within Nile tilapia populations studied in the present work is mainly due to sex polymorphism rather than individual variations and randomly introducing with mismating