The transposon Tn5 was used to map temperature-sensitive mutants of Myxococcus xanthus defective in aggregation (C. E. Morrison and D. R. Zusman, J. Bacteriol. 140:1036-1042, 1979). Seven of the eight mutants showing a similar terminal phenotype (rough) were found to be tightly linked. These mapped in a group of loci which we have designated aggR1, aggR2, aggR3, and aggR4. Temperature-sensitive mutants having a different terminal phenotype were not liked to aggR. A search through a group of nonconditional rough mutants indicated that a much lower proportion of these (1 of 35) mapped in aggR. Thus, aggR is probably only one of many sites which can lead to the rough phenotype when mutated. Localized mutagenesis was used to isolate nine additional aggR mutants. All mapped within aggR1, aggR2, or aggR3, and none was found outside this region. Thus, we have characterized a cluster of developmental genes which are needed for aggregation in M. xanthus. The localization of a Tn5 insert adjacent to this region makes possible further manipulation of these genes.