Adenovirus type 2 fibres in crystals appear to be significantly longer than found previously (accompanying paper). We therefore examined isolated fibre by electron microscopy and measured a length of 370 A, consistent with the length found in the crystals. The specific N-terminal structure of the fibre caused a heterogeneity in the length that may at least partially explain the values of 280 to 310 A published previously. Green et al. described a 15 amino acid repeat in the primary structure of the shaft of the fibre thought to be associated with the specific three-dimensional folding of the shaft. We compared the adenovirus type 2 (with 22 repeats) and type 3 (with 6 repeats) fibre lengths and derived a contribution of 13.2 A to the length of the shaft per 15 amino acid repeat. Specific morphological features of the fibre are discussed in relation to its amino acid sequence.