Techniques for the electron microscopic visualization of synaptonemal complexes were used to investigate the XY and XXY sex chromosomal systems in Scincella lateralis. Synaptonemal complex analyses revealed differences in the pattern of synapsis between the sex chromosomal and autosomal bivalents within each cytotype, and differences in sex chromosomal pairing between the cytotypes. Sex chromosomes of the XY cytotype synapsed in homology to form a bivalent. Sex chromosomes of the X1XY cytotype synapsed in a trivalent configuration, with localized regions of nonhomologous synapsis. These data support previous hypotheses that the X1XY condition is derived from the XY system by translocation of the Y chromosome to one homolog of the smallest macrochromosomal pair, and implicate centromeric inactivation in the fusion event.