In nematodes, sperm are amoeboid cells that crawl via an extended pseudopod. Unlike those in other crawling cells, this pseudopod contains little or no actin; instead, it utilizes the major sperm protein (MSP). In vivo and in vitro studies of Ascaris suum MSP have demonstrated that motility occurs via the regulated assembly and disassembly of MSP filaments. Filaments composed of MSP dimers are thought to provide the motive force. We have employed the yeast two-hybrid system to investigate MSP-MSP interactions and provide insights into the process of MSP filament formation. Fusions of the Caenorhabditis elegans msp-142 gene to both the lexA DNA binding domain (LEXA-MSP) and a transcriptional activation domain (AD-MSP) interact to drive expression of a lacZ reporter construct. A library of AD-MSP mutants was generated via mutagenic PCR and screened for clones that fail to interact with LEXA-MSP. Single missense mutations were identified and mapped to the crystal structure of A. suum MSP. Two classes of mutations predicted from the structure were recovered: changes in residues critical for the overall fold of the protein, and changes in residues in the dimerization interface. Multiple additional mutations were obtained in the two carboxy-terminal beta strands, a region not predicted to be involved in protein folding or dimer formation. Size fractionation of bacterially expressed MSPs indicates that mutations in this region do not abolish dimer formation. A number of compensating mutations that restore the interaction also map to this region. The data suggest that the carboxy-terminal beta strands are directly involved in interactions required for MSP filament assembly.