Randomly amplified polymorphic DNA (RAPD) analysis was used to examine geneticpopulation structure of ~iPenaeus monodon~i in Thailand using three primers (UBC268,UBC273 and UBC299). Five geographic samples; Satun, Trang and Phangnga located in theAndaman Sea and Chumphon and Trat located in the Gulf of Thailand were collected. Atotal of 53 RAPD fragments ranging from 200-1600 bp in size were scored. Eighty-eightRAPD patterns were observed for overall samples (30, 32 and 26 patterns from UBC268,UBC273 and UBC299, respectively). Considering private haplotypes and distributionfrequencies, ~iP. monodon~i from Trat were genetically different from the remainingsamples. The primer UBC268 generated a fragment of 260 bp in lengh found in almostall of Trat (90.2%) and Chumphon specimens (8%) but completely disappeared in thosefrom the Andaman Sea (Satun, Trang and Phangnga). Dendrograms constructed using unweight pair-group method with arithmeticaverage (UPGMA) allocated 5 geographic ~iP. monodon~i samples to two genetically isolatedgroups constituting of Trat and Satun, Trang, Phungnga and Chumphon. Geographicheterogeneity analysis using pseudo chi-square test based on Monte Carlo simulation,indicated highly significant genetic differences of ~iP. monodon~i between the AndamanSea and the Gulf of Thailand (P < 0.0001). Within the population in the Gulf ofThailand, Chumphon and Trat, there was also significant genetic differences (P (+,<)0.0001). Five geographic samples of ~iP. monodon~i could be regarded to be 3 separatedstocks; A (Trat), B (the Andaman Sea) and C (Chumphon)