Description:
<jats:p>Experiments were designed to study the effect of temperature on interpopulation crosses of <jats:italic>Gonium pectorale.</jats:italic> Both interpopulation and intrapopulation crosses were made at 10, 15, 20 and 25 C. Of the 34 populations studied, one‐third were definitely affected by temperature, showing that temperature is one factor in sexual isolation in at least some populations. Only two populations were completely incompatible with others. In some populations with both mating types, one mating type (the one designated “minus”) was found to be more compatible, possibly reflecting further genetic difference between mating types.</jats:p>