A novel phage lambda mutation, called dc10, which interferes with proper lambda head assembly has been isolated and characterized. Phage lambda carrying this mutation is (i) unable to form plaques at 30 or 37 degrees C but does so at 42 degrees C and (ii) unable to form plaques at 42 degrees C on pN-constitutive hosts. Both properties are due to dc10 since all phage revertants for one phenotype simultaneously lose the other phenotype and vice versa. The dc10 mutation has been mapped in the B gene and has been shown to be dominant over the corresponding wild-type product. At 30 degrees C the dc10 mutation results in the formation of abnormal petit lambda heads made up of pE, pB, pC, and pNu3. Under pN-constitutive conditions, the dc10 mutation results in the formation of abnormal petit lambda heads made of pE, X1, and X2 only. A model to explain the data is presented.