The sizes and shapes of different parts of pyrenes obtained from two diallel crosses of raspberry were influenced both by maternal effects and by interaction effects involving maternal and genetic factors. The relative importance of these effects depended upon the environment in which the crosses were made. For a diallel made under field conditions interactions with maternal factors were large but considerably reduced by applying auxin to the developing fruits. For a diallel made in a glasshouse maternal effects were more important and maternal interaction effects less so. Application of auxin to the fruits reduced endosperm or embryo size in some crosses and increased it in others. It is postulated that these responses were related to the endogenous content of growth substances, negative responses indicating their presence in supra optimal concentration, and that variations in the responses provide clues to the physiological factors which govern successful pyrene development.