Preliminary data showed evidence that mating type I and mating type II cells ofParamecium primaurelia,at the onset of the mating reactivity of the cultures, form different numbers of food vacuoles. The authors now investigate whether the differences in vacuole formation are visible at the beginning of culture growth, or whether they occur only later. Data collected during various stages in the early life of a clone, before mating reactivity develops, show higher food vacuole numbers per cell in mating type II cells than in mating type I cells. The difference in numbers of the two mating types disappears, and then the values reverse with starvation of the culture. These results are in agreement with previous data on the growth of complementary mating type cell lines, showing that different mean rates of growth characterize the two mating types during culture life.