We have studied the genetic relationships of echoviruses using nucleotide sequencing and hybridization analysis. The nucleotide sequence of the echovirus 11 (EV11) P2 and P3 regions, which encode the nonstructural proteins, was shown to resemble closely those of coxsackie B viruses (CBV) and coxsackievirus A9 (CAV9). EV11, CBV and CAV9 have a similar organization in the 3' non-coding region when compared to polioviruses and CAV21. In contrast, the 3' end of EV22 shares only minimal sequence homology with other sequenced enteroviruses, and the 3' non-coding region has a unique secondary structure. Thirty-three echovirus reference strains were tested by nucleic acid hybridization using cDNA probes from the genomes of EV6, 11, 18 and 22. It was shown that a great majority of the strains belongs to the same subgroup as serotypes 6, 11 and 18, whereas EV22 and EV23 are genetically not closely related to this major subgroup.