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Three cases of complementation between allelic mutants in heterocaryons of Neurospora crassa have been reported in which the individual homocaryons lack a specific enzymatic activity.'-3 More recent and apparently similar phenomena have been observed in which data relating to the involvement of a single enzyme are still lacking.4 6The present paper presents a detailed analysis of heterocaryon complementation between ad-4 mutants, each of which has impaired adenylosuccinase activity. The results suggest that the linear structure of both a gene and its products may be revealed by the pattern of interallelic heterocaryon complementation. The enzyme adenylosuccinase, found in wild-type extracts, catalyzes the splitting of adenosine monophosphate succinate (AMP-S) to adenosine monophosphate (AMP) and fumarate, as well as the splitting of the analogous purine precursor SAICAR to AICAR6 and fumarate." I Adenylosuccinase activity has not been found in appreciable amounts (less than 1 per cent of wild-type activity) in any of the ad-4 mutants tested, and partial restoration of activity has been detected in all cases examined in which complementation occurs. The degree of complernentation varies widely among the different mutant combinations; i. e., growth rates of the heterocaryons on minimal medium range from less than 0.2 to 4 mm/hr (wild-type rate), and enzyme activities range from less than 1 to 25 per cent of wild-type activity. Brief reports of certain of these results have been presented previously.7 8