Electron microscopy of spread spermatocytes and thin sections has been used to study the sex trivalent (XY1Y2) of the bat Artibeus lituratus. Pachytene spermatocytes in thin sections show an XY body with typical chromatin condensation that is connected to autosomal chromatin through a synaptonemal complex (SC). Microspread spermatocytes show three axes and two SC segments (a short SC and a long one) in the sex trivalent. The short paired region corresponds to synapsis between the 'original' X and Y pieces, while the long paired region corresponds to synapsis between the Y2 element and the homologous, autosomal piece of the compound X-chromosome. The length ratios of the three axes correspond to those of the three mitotic chromosomes, X, Y1 and Y2. The high packing of chromatin corresponds exclusively to the 'original' pieces of the X and Y elements, while the autosomal regions of the X and the Y2 axes are surrounded by autosomal-like chromatin. Thus, in this trivalent the formation of an XY body in the 'original' sex chromosomes is not inhibited by the presence of the autosomal pieces, and typical partial synapsis between the original X and Y elements is conserved. C-banding heterochromatin seems not to be the barrier preventing the spreading of heterochromatinization towards the autosomal piece in this trivalent.