Somatic chromosomes of both sexes and chromosome behaviour during spermatogenesis were studied in the aphid Schoutedenia lutea (van der Goot). Four long but unequal chromosomes in females were interpreted as X chromosomes (X1X1X2X2) with one member of an autosome pair attached to one X1, and the other member to one X2, so that the four long chromosomes were actually X1 + A, X1, X2 + A, X2. Males (normally XO in aphids) received X chromosomes corresponding in relative length to the two longest (X1 + A, X2 + A) in females. During spermatogenesis parallel pairing occurred in prophase 1 and the X1 and X2 chromosomes became associated via their autosomal segments. In anaphase I, the autosomal segment became detached from one of the X chromosomes and entered the non-viable (non-X-bearing) spermatocyte, while the viable spermatocyte received both X1 and X2 (either one of which still carried an autosome) and the haploid set of free autosomes. The consequences for sex determination and zygote formation of this unusual system are discussed; a stable chromosomal constitution for the zygote can be achieved only at the expense of considerable gamete wastage.