First meiotic division in females and males of Yponomeuta evonyrnellus (L.), Y. cagnagellus (Hiibner). Y. rnalinellus Zeller, Y. padellus (L), Y. rorellus (Hiibner) and Y. gigas Rebel was studied in the light microscope. In the heterogametic females the sex chromosomes were found to be associated in a sex chromosome trivalent. The sex chromosome trivalent, written AAWZ, consisted of a W‐chromosome translocated to an autosome to form an Aw‐chromosome, which was paired with the homologous auto‐some and the 2‐chromosome. In the homogametic males the two Z‐chromosomes were paired in a usual bivalent. The sex chromosome inheritance can be balanced in this way. The haploid chromosome number was n = 29 A + AAWZ in females and n = 30 A + ZZ in males, for all species examined. It is of interest to notice that the chromosome number of Y. rorellus earlier has been differently determined to be n = 29 for males. Especially in females the bivalents were observed to be nonhomologously associated at the telomeres during meiotic prophase.