Complex II of adult Ascaris suum muscle exhibits high fumarate reductase (FRD) activity and plays a key role in anaerobic electron-transport during adaptation to their microaerobic habitat. In contrast, larval (L2) complex II shows a much lower FRD activity than the adult enzyme, and functions as succinate dehydrogenase (SDH) in aerobic respiration. We have reported the stage-specific isoforms of complex II in A. suum mitochondria, and showed that at least the flavoprotein subunit (Fp) and the small subunit of cytochrome b (cybS) of the larval complex II differ from those of adult. In the present study, complete cDNAs for the iron-sulfur subunit (Ip) of complex II, which with Fp forms the catalytic portion of complex II, have been cloned and sequenced from anaerobic adult A. suum, and the free-living nematode, Caenorhabditis elegans. The amino acid sequences of the Ip subunits of these two nematodes are similar, particularly around the three cysteine-rich regions that are thought to comprise the iron-sulfur clusters of the enzyme. The Ip from A. suum larvae was also characterized because Northern hybridization showed that the adult Ip is also expressed in L2. The Ip of larval complex II was recognized by the antibody against adult Ip, and was indistinguishable from the adult Ip by peptide mapping. The N-terminal 42 amino acid sequence of Ip in the larval complex II purified by DEAE-cellulofine column chromatography was identical to that of the mature form of the adult Ip. Furthermore, the amino acid composition of larval Ip determined by micro-analysis on a PVDF membrane is almost the same as that of adult Ip. These results, together with the fact, that homology probing by RT-PCR, using degenerated primers, failed to find a larval-specific Ip, suggest that the two different stage-specific forms of the A. suum complex II share a common Ip subunit, even though the adult enzyme functions as a FRD, while larval enzyme acts as an SDH.