Genetic variation among and within five generations of an inbred commercial captive line of Litopenaeus vannamei and genetic distance among them were evaluated by random amplified polymorphic DNA (RAPD), using descriptive and genetic similarity analyses for dominant markers at single- and multi-populational level. One hundred individuals were analyzed and 56 polymorphic loci were identified with a set of six primers screened in a painel of 38 decamer primers tested. The values obtained for F5, F6, F7, F8 and F9 generations revealed a progressive increasing of genetic similarity throughout the five stocks, confirmed by Nei genetic diversity analysis, which values decreased from 0.27 to 0.22 at F5 and F9, respectively. Significant allele frequency differences were observed at most analyzed loci. Strong correlation (R2 = 0.9845) was observed between genetic similarity and generation time, and genetic similarity could reach closer to 100% at 18-19 generation of this line.