Technical Abstract: Dry bulb onions are poor competitors and volunteer potato is commonly observed in the western U.S. and difficult to manage in the crop. In order to improve the understanding of onion and weed interactions, relationships were quantified among volunteer potato density, onion yield, and volunteer potato tuber production using hyperbolic or linear models. Onion yield losses due to volunteer potato interference occur at densities commonly observed in the field. Percent crop yield loss as weed density approaches zero (I parameter) was 165 and initial volunteer potato density as low as 1 plant per 14.9 m-2 resulted in 10% crop yield loss. Asymptotic yield loss (A) was 100% and achieved with some 4 volunteer potato plants m-2. Volunteer potato competition limits onion bulb size, resulting in a lower quality, less valuable crop. Volunteer potato tuber density and biomass increased linearly with initial weed density as high as 8 volunteer potatoes m-2. Onion yield loss from volunteer potato competition occurs to a greater extent and at a lower weed density than demonstrated in previous research on small seeded annual weed species.