We studied four populations of the pierid butterfly Eucheira socialis Westwood, located in the Mexican states of Sinaloa, Durango, Mexico, and Oaxaca. We found extreme geographic variation in spermatogenic abnormalities, including a variable number of chromosomes, irregular pairing of chromosomes, lagging chromosomes in metaphase I, lagging chromosomes in anaphase I, the production of micronuclei, and the failure of some spermatids to condense before maturation. We also present evidence suggesting the production of aneuploid sperm. These abnormalities correlate with previously reported male biases in the primary sex ratio of two of these populations. The population with the greatest bias in sex ratio also exhibited the highest frequencies of spermatogenic abnormalities. Because butterflies have female heterogamety and Eucheira females are XO, a possible mechanism linking the sex ratio biases and these abnormalities is nondisjunction of the X chromosomes during spermatogenesis, yielding disomic X sperm. We conclude that the most parsimonious explanation for the meiotic abnormalities in E. socialis is the presence of supernumerary chromosomes, which are associated with sex ratio biases in other systems.