An artificial crossing program involving 8 annual form‐series within Hypecoum sect. Hypecoum (Papaveraceae) was carried out. In most combinations between different species, hybrid inviability due to disparity between parental genomes seems to be the most important crossing barrier: chlorotic progeny was produced, or hybrids with low vitality, few and sterile flowers or dwarfishness occurred. The three form‐series considered as different subspecies of H. procumbens seem rather to be separated by hybrid break‐down mechanisms in advanced generations, expressed as malformation of organs, and often pronounced decrease in fertility. Crossings between populations within the same form‐series usually yielded vigorous and highly fertile progeny. The breeding relationships within the group were found to be well correlated with discontinuities in morphological characteristics. The disparity between the genomes of the different form‐series are probably mainly due to gene mutations and cryptic structural alterations and not to major structural poly