1. Eleven genetic factors are described in the garden Iceland poppy,Papaver nudicaule; five of them control flower colour, six control other characters. In addition, notes are given on the inheritance of six other variations the exact genetic nature of which is not yet established. 2. In the wild,P. nudicaule exists as a complex of intergrading forms. Much of the variation among them appears to correspond to that found in garden strains, although most if not all the horticultural varieties arose by mutation under cultivation. 3. A linkage group of three factors has been found. A fourth factor is completely linked to one of these three; alternatively, it may be an allele of it. In one large backcross, crossing over in this group was almost completely inhibited, probably by an inversion. Data on the independence of several other factors are given. 4. Self-incompatibility in the Iceland poppy is controlled by a system of oppositional factors, belonging to a series of multipleS alleles, similar to that inNicotiana, Antirrhinum, clover, and other plants. Self-fertility, due probably to anS F allele has been found. 5. The anthocyanin chemistry of flower colour gene interactions is in the main not straightforward. 6. With few exceptions, single factor ratios show much heterogeneity and departure from expectation. This seems to be due to several causes operating simultaneously.