Females of Phytoseiulus persimilis deposit eggs with a female-biased sex ratio under an ample prey condition, but deposit smaller eggs with an unbiased sex ratio under a poor prey condition ; in this condition male eggs and female eggs are produced alternately. The relationship between the size of the eggs and the life history characteristics of the immature and adult stages was examined by rearing two different sized eggs, which are laid under the ample and poor prey conditions, to clarify the changes in sex ratio and the alternate production of sexes. Female offspring produced under the poor prey condition developed more slowly than those produced under the ample prey condition did, and the former consumed fewer prey eggs daily during the nymphal stage. However, the total prey consumption during the entire nymphal stage and the body size of adult offspring showed no differences between the two prey conditions. In addition, females from both conditions had similar reproductive abilities. Male offspring showed the same trend in developmental and preyconsumption rates, whereas adult males from the poor prey condition showed a lower fertilization potential than those from the ample prey condition. The decreases in the developmental rates of both sexes and in the reproductive abilities in males are disadvantageous characteristics, but the alternate production of sexes may compensate and assure female offspring of insemination even under the poor prey condition.