A pseudolysogenic Mycobacterium chelonei and its phage phi630 are described. Phage phi630 is the first mycobacteriophate reported to be resistant to the nonpolar solvents chloroform, dioxan and diethyl ether. The phage had a latent period of 75 min, a rise period of 90 min and a burst size of 5I. Evidence is presented for host modification and restriction. Phage phi630A, grown on host strain M. chelonei F-630 Rg, plated on the alternative host M. smegmatis ATCC607 with an efficiency of plating (e.o.p.) of 10(-5). Phage phi630B, grown on host M. smegmatis, plated with an e.o.p. of 10(-5) on the alternative host F-630 Rg. Phages phi630A and phi630B absorbed equally well on their alternative hosts and on their indicator host strains. The progeny of plaques from initial platings on the alternative host, when grown in the alternative host, exhibited a marked reduction in e.o.p. on their original host.