Necessary stereochemical requirements for an amino acid sequence segment to fold into an alpha-helix-turn-alpha-helix supersecondary structure are presented in sequence template form. The usefulness of the template is illustrated by alpha-helix-turn-alpha-helix predictions consistent with experimental data from the large T antigens of two polyoma viruses, simian virus 40 (segment 143-165) and mouse polyoma virus (segment 297-319), and the yeast transcription activator GCN4 (segment 256-278).