Hypothesis: Genetic variation for the primary sex ratio is widespread in a copepod with polygenic sex determination. Cytoplasmic sex ratio distorters (e.g. Wolbachia and microsporidians) influence the primary sex ratio in this copepod. Organism: The intertidal copepod, Tigriopus californicus; six populations from Vancouver Island, British Columbia. Study site: Quantitative genetics experiment in the laboratory. PCR and antibiotics experiment to test for the presence of cytoplasmic sex ratio distorters. Results: Genetic variation for the primary sex ratio was found in five of the six populations surveyed. The primary sex ratio was paternally transmitted. There was no evidence that Wolbachia or microsporidians influenced the primary sex ratio of T. californicus.