A simple method for determination of the early stages of nymphs of the cricket, Gryllus bimaculatus, was presented. The width of the head-capsules was found to be a useful value to determine the nymphal stages. On this evidence, the testes from 3rd- to 6th-instar nymphs were investigated and histologically analyzed. The number of spermatogonia rapidly increased between the 3rd- to 5th-instars. Primary spermatocytes entered into meiotic prophase at the latest 2 days after ecdysis into the 5th-instar.