Dictyostelium mucoroides plasmids Dmp1 and Dmp2 are naturally occurring compatible members of the Dictyostelium Ddp1 plasmid family found in the same wild isolate strain. The nucleotide sequences of Dmp1 (5983 bp) and Dmp2 (6018 bp) are 74% identical and each carries open reading frames (ORFs) similar to the G1 and G5 ORFs of the Dictyostelium discoideum plasmid Ddp1. The predicted protein product of the Ddp1 G1 ORF is 49% similar to that of the Dmp1 G1-like ORF and 52% similar to that of the Dmp2 G1-like ORF. For the G5 and G5-like ORFs the corresponding values are 47 and 43%, respectively. The G1 and G5 ORFs of Ddp1 are transcribed during both vegetative growth and development of the asexual fruiting body. The G1-like ORF of Dmp2 is expressed in vegetative and developing cells, while that of Dmp1 appears to be expressed mostly in developing cells. The G5-like ORFs of Dmp1 and Dmp2 are expressed in both vegetative and developing cells. Dmp1 and Dmp2 differ from Ddp1 in (1) lacking homologs for the Ddp1 G2/G3/D4, G4/D5, D1/D3, and D2 ORFs; (2) containing multiple copies of a 173 bp direct repeat; and (3) having a different orientation of the G1 ORF relative to the G5 ORF. These findings suggest that the basic replicon unit of the Ddp1 plasmid family is composed of an origin of replication coupled to G1-like and G5-like genes. The additional ORFs and direct repeat elements in Ddp1, Dmp1, and Dmp2 may provide accessory functions beneficial to plasmid maintenance. Shuttle vectors based on Dmp1 or Dmp2 replicate in D. discoideum transformants.