The barley mutant, poly-row-and-branchedspike (prbs) showed altered inflorescence morphology:complete conversion of the rudimentary lateral spikelets intwo-rowed barley into fully developed fertile spikeletssimilar to the six-rowed phenotype, and additional spikeletsin the middle of spike. Moreover, branched spikes emergedin progeny from a cross between the mutant and a sixrowedbarley cultivar. Morphological observation of thedevelopment of immature spikes of the mutant anddescendants with branched spikes showed that the Prbsgene is involved in spikelet development in the triplemoundstage. In mutant prbs, new meristems initiated atthe flanks of lateral spikelets and middle spikelet meristemswere converted to branch meristems, developing branchedspikes. These observations suggested that the Prbs geneplays a crucial role in spikelet initiation and identitymaintenance. The Prbs gene may be an important modifierin inflorescence differentiation from a panicle into a spike. The branched spikes emerging in hybrids from a crossbetween the mutant and six-rowed barley cultivar were notconferred by the gene vrs1 or Int-c, which decide spikemorphology in six-rowed barley. These results imply thatalthough six-row genes vrs1 and Int-c and prbs have similareffects on lateral spikelet development, they have differentfunctions in branched spikes. The Prbs gene wasmapped to chromosome 3H between SSR markerBmag0023 and marker Cbic60 at a genetic distance of 3.3and 5.4 centimorgans (cM), respectively.