Technical Abstract: Chemicals produced by mustard (Brassica) crops as they are tilled into the soil have been shown especially in laboratory trials to suppress soil-dwelling organisms, including fungal pathogens, nematodes, and some insect pests of crops. Here, I tested in the field whether an autumn mustard crop grown between a wheat-potato rotation led to reductions in numbers of Pacific coast wireworm and damage by wireworms to tubers. Mustard was planted in late summer and tilled into the soil in October preceding a potato crop the following season. The mustard had no effects on spring densities of Pacific coast wireworm or subsequent damage to harvested tubers caused by wireworms. Damage rates were higher for large tubers than small tubers, in both mustard and control plots. There was a slight but statistically significant drop in tuber yield associated with the mustard cover crop.