On average, hermaphroditic proglottids of 10-month-old Hymenolepis diminuta proceeding from WMS il1 contained more testes when tapeworms were cultured in crowded populations (44-48 individuals) than in lower density (4-6 individuals). Furthermore, one average-sized testis from a tapeworm of the crowded population accounted for a relatively greater part of proglottid volume than did one in tapeworms from the low-density populations. These same populations - both low-density and crowded - included tapeworms with unilateral positioning of genital ducts and pores, or with changes in the positioning of these organs in strobilae. This variability is correlated positively with the number of proglottids of type 2p1a (containing two testes on the poral side and one on the aporal), and negatively with the numbers of types 1p2a, 0p3a and 1p3a (P<0.01).