Summary Mycoplasmatales virus-laidlawii 1 (MV-L1) produced plaques on lawns of 11 out of 17 strains of Acholeplasma laidlawii but not on 12 other mycoplasma species examined. Clones of A. laidlawii resistant to lysis by the virus could readily be obtained from survivors of the susceptible bni strain. These resistant clones carry virus apparently serologically similar to MV-L1 as does the susceptible bni strain itself. The virus was sensitive to the action of u.v. light and chloroform but relatively insensitive to ether. Nonidet-P 40, heat and pH levels of between 8 and 10. MV-L1 passed readily through Millipore VS filters of 25 nm. pore diameter with loss of titre of 0.5 log p.f.u./ml. or less. In one-step growth experiments the latent period was between 30 and 60 min. and the burst size varied from 4 to 213. Maximum yield was obtained when virus was added early in the logarithmic stage of acholeplasma growth. MV-L1 appeared to cause gradual lysis of the acholeplasma culture.