Abstract The effects of mutations in the morphogenetic genes of the head have been examined by electron microscopic analysis of lysates of phages carrying mutations in genes Nu1, A. W, B, Nu2, C, Nu3, D, E and F, and phages carrying multiple mutations of gene B and other head genes. The only head-related structure produced by phage with mutations in genes Nu1, A or C is petit λ. Gene E is the only head gene essential for the production of petit λ. The products of genes W, D and F are not required for the formation of heads and the products of genes W and F are not essential for the formation of DNA filled heads. The product of gene E but not the products of genes A, W, B, C, D and F (and perhaps Nu3) are needed for the synthesis of monsters. Polyhead synthesis is depended on the products of genes E and C, but not of genes A, W, B, D and F. A model is proposed for the assembly of the gene E product.