Bacteriophage phi29 is a small, morphologically complex, virus with a DNA of molecular mass 12 X 10(6). The most likely structure of the head of phi29 consists of two fivefold symmetric end-caps based on T = 1 icosahedral symmetry, separated by an equatorial row of 5 hexamers. The eighteen genes identified in phi29 genome have been mapped and, in some cases, the gene products have been identified. Five linked genes, four coding for structural proteins (G, A, E, H) and one coding for a non-structural protein (J), are essential to determine the normal shape of the capsid. Protein pJ may be a scaffolding protein. An account of the effects of mutations in phi29 genes is given.