Little information is available on genetic variation within and between populations of pest cockroaches. In this study, intraspecific HindIII polymorphism was investigated in the German cockroach, Blattella germanica (Linnaeus) (Dictyoptera, Blattaria: Blattellidae), using restriction fragment length polymorphisms (RFLP) of the non-transcribed spacer (NTS) region of ribosomal DNA (rDNA). Individual male insects were collected from infestations at three different pig farms. Each population was characterized by HindIII restriction fragment frequencies and haplotype (a particular X-chromosome pattern) frequencies. The inheritance of the X-chromosome HindIII rDNA patterns over 12 generations (3 years) follows Mendelian patterns, and the stability of this polymorphic marker indicates infrequent genetic recombination of variable sites. Although pairwise genetic distance measures were uncorrelated with geographical distance, the pattern of genetic differentiation of the three cockroach populations suggests that human-mediated transport of cockroaches is an important force in shaping the population genetic structure of cockroach infestations, at least at the regional scale of 10-100 km. Sequence variation in the ribosomal NTS is a useful marker, and RFLP of rDNA is a simple, robust and reproducible technique for differentiating recently diverged cockroach populations.