Sex chromosome morphology of eight Lepidoptera species was studied, exploiting predominantly the pachytene stage when chromosomes display a remarkable chromomere pattern. Six species had a WZ/ ZZ sex chromosome system, one species a W1W2Z/ ZZ system and one species was of the Z/ZZ type. Much like XY chromosomes in groups with male heterogamety, the lepidopteran sex chromosomes showed various degrees of structural differentiation. Differences between Z and W chromomere patterns ranged from undetectable to obviously non-homologous. A common property of the W chromosomes (the W1 in the W1W2Z/ZZ system) was the possession of a block of heterochromatin. The heterochromatin block comprised a small or a large segment of the W or even the entire W, depending on the species. Segments with apparent structural homology are evolutionarily young parts of the sex chromosomes-recently fused autosomes that have not had sufficient time for differentiation. The 'primitive' lepidopteran species Micropterix calthella had a Z/ZZ sex chromosome system. This supports the hypothesis that the lepidopteran W chromosome came into being at the base of the 'advanced' Lepidoptera; it was presumably an autosome whose homologue fused to the original Z chromosome.