Gillichthys mirabilis Cooper (mudsucker, long-jawed goby) collected at Baja California, Mexico, near Scammon Lagoon, Salton Sea, Seal Beach and Newport Bay, and San Francisco Bay, California were examined for helminths. The monogenetic trematode, Gyrodactylus sp., was found on the gills of fish from all localities except the Salton Sea. A small digenetic trematode was recovered from the posterior intestine of fish from Baja California and is described as Lecithaster minimus sp. n. Encysted trematodes and larval cestodes also were found in fish from Baja California and Seal Beach. An adult acanthocephalan, Microsentis wardae Martin and Multani, 1966, was found in fish from Baja California. A larval acanthocephalan was recovered from the coelom of hosts from Seal Beach. The nematode, Vasorhabdochona cablei Martin and Zam, 1967, was found in certain blood vessels of fish from Baja California and Seal Beach. Another nematode, Spirocamallanus pereirai (Annereaux, 1946) was found in the intestine of fish from all localities except the Salton Sea.