Four fish species; two fresh water fish (Oreochromis niloticus L. and Tialpia zillii L.), and two marine fish (Liza ramada L. and Mugil cephalus L.) were used in this study. These species were collected from Damietta and Port Said governorate, respectively. The aims of this study were to: a) Study the genetic variation within and between freshwater and marine water fish using both fingerprinting (e.g. fluorescence AFLP) and partial sequencing of growth hormone gene for each species. b) Isolate and sequence intron 5 of growth hormone (GH) gene to study inter specific variation among freshwater and marine fish. c) Test the genetic variation for being associated to the species habitats. 1. Amplified Fragment Length Polymorphism (AFLP) fingerprint 2. Growth hormone (intron5) gene Sequencing