Two SH3 domain‐containing cytosolic components of the NADPH oxidase, p47^phox and p40^phox, are shown by analyses of their sequences to contain single copies of a novel class of domain, the PX (phox) domain. Homologous domains are demonstrated to be present in the Cpk class of phosphatidylinositol 3‐kinase, S. cerevisiae Bem1p, and S. pombe Scd2, and a large family of human sorting nexin 1 (SNX1) homologues. The majority of these domains contains a polyproline motif, typical of SH3 domain‐binding proteins. Two further findings are reported. A third NADPH oxidase subunit, p67^phox, is shown to contain four tetratricopeptide repeats (TPRs) within its N‐terminal Racl^GTP‐binding region, and a 28 residue motif in p40^phox is demonstrated to be present in protein kinase C isoforms τ/λ and ζ, and in three ZZ domain‐containing proteins.