Pachytene chromomere maps of early and mid/late mouse spermatocytes have been prepared which permit exact identification of each bivalent. The average total number of chromomeres on the autosomal bivalents was 248 in the early cells and 184 in the mid/late. There was close correspondence between early and mid/late chromomeres in 122 locations. Comparisons of early pachytene chromomeres with published prometaphase dark G bands revealed 1.6 more chromomeres in the meiotic autosomal bivalents, with close correspondence of larger chromomeres and major mitotic bands. Fewer chromomeres were found in pachytene spermatocytes than had been seen in a previous study of pachytene oocytes. Comparisons of chromomeres of spermatocytes and oocytes revealed several differences.