The relationship between body size and reproduction was investigated for two laboratory strains of Podisus nigrispinus (Dallas). A strain originating from Surinam and maintained for about 15 years under laboratory conditions, did not demonstrate any significant relationship between body length and fecundity. A second strain originated from Brazil where it had been reared in the laboratory for several generations but under different conditions of climate, food and housing. In the first generation of the Brazilian strain, a significant positive relationship was found between body length and total number of eggs. Results suggest that colonization might act initially in favour of a positive body size/fecundity relationship. In contrast, females of the fifth and tenth generations, like those of the Surinamese strain, showed no significant relationship between body length and reproduction. Fecundity of the tenth generation of the Brazilian strain, with an average of 317 eggs per female, was significantly greater than that or the first and fifth generation, with 220 and 243 eggs per female, respectively. The strain from Surinam, with 545 eggs per female, had a markedly higher reproductive capacity than any generation of the Brazilian strain. Adult body weights of different generations of the Brazilian strain increased consistently from the first to the tenth generation, indicating an adaptation to the new environment. Females and males of the Surinamese strain lived longer than those of the Brazilian one. Besides differences related to the geographical origin of the strains, greater fecundity and longevity in long-term laboratory strains of P. nigrispinus may reflect selective adaptation to food and climatic conditions in the laboratory environment.