A series of aggregation-deficient (aggregateless) mutants were isolated in genetically marked haploid strains of the cellular slime mould Dictyostelium discoideum. Diploids were produced from pairs of such haploid mutants by a fusion system based on this organism's parasexual cycle. The diploids were isolated from the haploids by using complementation of non-allelic growth-temperature-sensitive mutations and selection at the restrictive temperature. Complementation between the aggregateless mutations uas then assessed in 419 diploids so formed. The non-complementing aggregateless mutations fell into five complementation groups (ago A, B, C, D, and E) and a dominant aggregation class that allowed little or no aggregation when present in a diploid with any of the other mutations tested or the parental wild type. Complicating factors, including partial dominance, multiple mutations, and possible interallelic conplementation, are discussed. Data on the linkage of the aggregateless mutations was obtained by using recessive drug resistance mutations on three linkage groups to segregate haploids from the diploids. Calculations from our results suggest a genetic complexity of about 50 genes that are specific and essential for aggregation.