The production of a small low-abundance protein encoded by the large dsRNA genome segment of bacteriophage phi 6 in extracts of phage-infected Pseudomonas phaseolicola has been reported previously. Construction and analysis of a phage containing a nonsense mutation in this gene, designated gene 14, indicates that its product is not essential for growth in the laboratory. However, since early protein synthesis is delayed, burst sizes are reduced, and plaques are smaller than wild type, P14 is needed for optimal phage development. (C) 1996 Academic Press, Inc.