DNA replication in Escherichia coli and therefore bacterial proliferation relies upon the efficient functioning of the DnaB helicase. The toxin protein Kid from the plasmid-stability system parD encoded on plasmid R1 of E. coli is thought to target and block DnaB-dependent DNA replication. The toxicity of Kid is antagonized through interaction with the Kis antidote protein and the resultant complex can then act as a transcriptional regulator for the parD system. Crystals of selenomethionine-incorporated Kid have been obtained by the hanging-drop vapour-diffusion method using potassium phosphate as the precipitant. The crystals belong to the monoclinic system, space group P2(1), have unit-cell parameters a = 32.9, b = 45.0, c = 64.4 A, beta = 96.2 degrees and diffract to a d(min) of better than 1.8 A on a synchrotron-radiation source. The determination of the structure of Kid will permit a better understanding of its interactions with DnaB and Kis and allow the evolutionary relationships of Kid to other toxins of plasmid and chromosomal origin to be explored.