Summary Five isolates of Mycoplasmatales virus-laidlawii 2 (MV-L2) derived from bovine strains of Acholeplasma laidlawii were shown to differ in host range, plaque morphology and neutralization tests with MV-L2 antiserum. Cross-testing using virus resistant clones of A. laidlawii confirmed the heterogeneity of this group. Adaptation of viruses to sub-optimal hosts was demonstrated.