We present a detailed comparison of Papaver californicum and Stylomecon heterophylla, which earlier were found to be sister species and most closely related to Meconopsis cambrica Papaver s.str. from western Eurasia. The two species of winter annuals differ mainly in the shape of their distal cauline leaves, coloration of petals and staminal filaments, and most notably morphology of the gynoecium and capsule, with Papaver californicum having a sessile stigmatic disc and Stylomecon heterophylla having a distinct style. They were earlier found to differ in ploidy, with chromosome numbers of 2n  =  28 (Papaver californicum) and 2n  =  56 (Stylomecon heterophylla). Mapped distributions of the two species indicate that the range of S. heterophylla encompasses and exceeds that of P. californicum; both are known only from the California Floristic Province except for one collection of S. heterophylla from central Baja California. Whereas Papaver californicum is most commonly found in burn localities in the first wet season after fire, Stylomecon heterophylla is active under a broader range of environmenal conditions and often occurs in habitats that appear to be somewhat more mesic. Both species are self-compatible and autonomously self-pollinating. Experimental hybridization resulted in well-developed but entirely sterile hybrids; no hybrids are known from nature. Based on these and earlier findings, we conclude that Stylomecon heterophylla is best treated in Papaver, as P. heterophyllum.