The tendency for axillary head (AH) development in Brassica oleracea (Capitata group) was shown to be a heritable characteristic. In 11 crosses between high and low AH lines carried at least through the F2 generation, AH was recessive and largely controlled by one or a few genes. In other crosses, in which only the F1 generation was studied, AH tendency usually resembled that of the parent having a low level of AH, but in certain crosses was intermediate or closer to the high scoring parent. Expression of AH was continuous and apparently influenced by modifying genes and the environment. Broadsense heritability estimates were high, but narrowsense heritability estimates were very low indicating dominance effects. No consistent or apparently important differences were found between reciprocals. The weight of AH was not related to yield of main heads in F2 plants.