The Neotropical ant Ectatomma tuberculatum (Olivier) shows a polymorphic social organization with strictly monogynous or facultative polygynous populations along its distribution range. The causative factors of these variations in social organization are unknown but biotic and abiotic environmental factors might explain the differences. In particular, recent studies have suggested that parasites and parasitoids could have an impact on host colony phenotype. Here, we report on eucharitid wasps and mermithid nematodes that attack the only known Mexican polygynous population of E. tuberculatum in Veracruz, Mexico. The results were compared with those previously obtained from a monogynous population in Chiapas, Mexico; the aim being to investigate environmental influences on colony social organization. The rather similar prevalence of parasitism between the 2 populations suggests that the hypothesis of an increased resistance to parasites and parasitoids as one of the factors responsible for the emergence of polygyny in E. tuberculatum populations does not apply. More likely, other kinds of environmental stress or constraints facilitate the development of polygyny, such as nest site limitations, risks of dispersal and/or high rates of predation.