Phacelia stebbinsii Constance & Heckerd, P. marcescens Eastw. ex J.F.Macbr., and P. quickii J.T.Howell are morphologically similar annual taxa endemic to the west side of the Sierra Nevada in California. Experimental crosses show P. quickii is reproductively isolated from the other two species, and that P. marcescens and P. stebbinsii readily produce fully fertile F₁ and S₂ hybrid progeny, but typically only when P. marcescens is the carpellate parent. The presence of geographic separation, habitat differences, unilateral crossability, pollen certation, and problems with cotyledon emergence in hybrid seedlings provide evidence P. stebbinsii is distinct from P. marcescens and they are maintained as separate species. Flower and fruit characters used to distinguish these three species from each other appear to be related to differences in seed dispersal and seedling establishment in different habitats.