Plasmids pIKΔ and pIKΔ-erm have recently been developed as mycoplasmal cloning vectors. In this report, we demonstrate that these plasmids can replicate in Mycoplasma capricolum, a mycoplasmal species for which transformation had not previously been characterized. Both plasmids are stably maintained at a higher copy number than in their parental species. Mycoplasma mycoides subsp. mycoides. We have also examined the possibility of one or more restriction-modification systems affecting transformation frequencies in both species. © 1994 Academic Press, Inc.