Endocrine disruptors have well-known effects on vertebrate population sex ratios, but a variety of chemicals can also affect sex ratios of invertebrates. We tested whether sex ratios of fleas (Ceratophyllus idius Jordan and Rothschild) living in the nests of tree swallows (Tachycineta bicolor Vieillot) were affected by exposure to environments containing different chemicals. Fleas were collected from nests adjacent to sewage lagoons that received unknown chemical inputs, from nests fumigated with insecticidal vegetation (yarrow, Achillea millefolium L.), from nests treated with both commercial flea powder and diatomaceous earth, and from control nests. The proportion of males was lower in nests treated with both commercial flea powder and diatomaceous earth than it was in nests around the sewage lagoon or in nests to which yarrow had been added. These results suggest potentially novel effects of chemicals on invertebrate populations that would not be revealed by, for example, LD₅₀s.