Seeds of 37 species of Calochortus were examined using scanning electron microscopy. Two major seed forms were observed. "Calochortoid" seeds are irregular in shape with roughly tetragonal to irregular testa cells lacking secondary thickenings in the outer cell walls. "Mariposoid" seeds are flattened and lenticular in outline with roughly hexagonal to irregular testa cells with secondary thickenings in the outer cell walls. Calochortoid seeds are characteristic of sect. Calochortus and of one species of sect. Cyclobothra (C. barbatus). Mariposoid seeds are characteristic of sect. Mariposa and of subsect. Weediani of sect. Cyclobothra. Both gross morphological evidence and SEM data indicate that sect. Cyclobothra may be polyphyletic. Subsection Weediani appears to have greater affinity to sect. Mariposa than to other members of sect. Cyclobothra. The remaining species in sect. Cyclobothra need further study to determine their affinities. Calochortus Pursh comprises approximately 60 species distributed in western North America from British Columbia to Guatemala. Calochor- tus differs from other Liliaceae in having a short or obsolete style, a septicidal capsule, sepaloid outer perianth segments, and polygonum em- bryo sac formation (Dahlgren et al. 1985; Own- bey 1940), and has been placed by Dahlgren et al. (1985) in its own family, the Calochortaceae. Its affinities with genera in Liliaceae are uncer- tain. Although Calochortus bears a superficial re- semblance to Fritillaria L., Ownbey (1940) sug- gested that it was actually more closely related to Tulipa L. Calochortus comprises three sections: Calo- chortus, Mariposa, and Cyclobothra (Ownbey 1940). Section Calochortus is distributed throughout the western United States and its center of diversity is in the Pacific Northwest. It has a uniform base chromosome number of x = 10; short 3-winged capsules; membranaceous bulb-coats; conspicuous, persistent basal leaves; and irreg- ular seeds. Section Mariposa is also distributed throughout the western United States, but its center of diversity is in central and southern California. It has a variety of base chromosome numbers (i.e., x = 6-10); oblong to linear, 3-angled fruits; membranaceous bulb-coats; basal leaves that wither prior to anthesis; and flattened lenticular seeds. Section Cyclobothra is distributed in southern California and Mexico with a center of diversity in northern Mexico.