SEVERAL bacteriophages use the Escherichia coli GroES and GroEL chaperonins for folding and assembly of their morphogenetic structures1. Bacteriophage T4 is unusual in that it encodes a specialized protein (Gp31) that is thought to interact with the host GroEL and to be absolutely required for the correct assembly of the major capsid protein (Gp23) in vivo2-4. Here we show that despite the absence of amino-acid sequence similarity between Gp31 and GroES5,6 Gp31 can functionally substitute for the GroES co-chaperonin in the morphogenesis of bacteriophages lambda and T5, the in vivo and in vitro chaperonin-dependent assembly of ribulose bisphosphate carboxylase (Rubisco), as well as overall bacterial growth at the non-permissive temperature. Like GroES, the bacteriophage Gp31 protein forms a stable complex with the E. coli GroEL protein in the presence of Mg-ATP and inhibits the ATPase activity of GroEL in vitro.