Pentachaeta lyonii A. Gray is a state- and federally-listed endangered species, endemic to heavily invaded southern California grasslands. Recent population extirpations resulting in a decrease in range size have prompted investigation into the effects of invasive annual plants on this species. The goals of this study were (1) to examine the impacts of competition from non-native species from three different functional groups (annual grasses, early-season forbs, and late-season forbs) on P. lyonii success in the field and in pots, (2) to determine which non-native species/functional groups have the greatest competitive effect on P. lyonii, and (3) to evaluate the environmental conditions that contribute to the displacement of P. lyonii by non-native plants. In the field, at two sites over two years, control plots were paired with plots in which non-native competitors were clipped at the soil surface. In pots, individual P. lyonii plants were grown in competition with all three groups of non-native competitors at both high and low density. In both the field and in the pots, all three non-native plant groups had negative effects on P. lyonii reproductive potential, with Centaurea melitensis L. having the greatest effect. The effects on P. lyonii height were variable among non-native competitors and between years. Comparisons made of environmental features of sites where P. lyonii has been extirpated to those where it persists suggested that the presence of annual grass is associated with P. lyonii extirpation. Management of P. lyonii presents a challenge considering the tendency of this species to coexist with non-native annual plants due to their common disturbance-dependence, and the ubiquity of European annuals in P. lyonii habitat.