Two glutathione S-transferase isozymes from the mosquito Anopheles dirus (AdGST1-3 and AdGST1-4) from an alternately spliced gene family have been expressed, purified and crystallized. The isozymes share an N-terminal domain derived from a single exon and C-terminal domains from unique exons. Despite the high level of sequence identity (64% overall), the two isozymes crystallize in different space groups, the 1-3 isozyme in P3(1)21 or P3(2)21 (unit-cell parameters a = 49.9, c = 271.8 A at 100 K) and the 1-4 isozyme in P4(1) or P4(3) (unit-cell parameters a = 87.8, c = 166.1 at 100 K). Determination of these structures will advance our understanding of how these enzymes inactivate pesticides and the structural consequences of alternate splicing.