The recessive labile locus mapped on chromosome 5HL causes irregular spikelet fertility and controls floret development as well as row-type in barley. The labile-barley displays a variable number of fertile spikelets at each rachis internode (0-3 fertile spikelets/rachis internode) which is intermediate between that observed in two- or six-rowed types. Previous re-sequencing of Vrs1 in 219 labile-barley (Hordeum vulgare L. convar. labile) accessions showed that all carried a six-rowed specific allele. We therefore hypothesized that this seemingly random reduction in spikelet fertility is most likely caused by the labile (lab) locus, which we aimed to phenotypically and genetically define. Here, we report a detailed phenotypic analysis of spikelet fertility in labile-barleys in comparison to two- and six-rowed genotypes using scanning electron microscopy analysis. We found that the first visible morphological deviation occurred during the stamen primordium stage, when we regularly observed the appearance of arrested central floral primordia in labile but not in two- or six-rowed barleys. At late stamen and early awn primordium stages, lateral florets in two-rowed and only some in labile-barley showed retarded development and reduction in size compared with fully fertile lateral florets in six-rowed barley. We used two F2 mapping populations to generate whole genome genetic linkage maps and ultimately locate the lab locus as a recessive Mendelian trait to a 4.5-5.8 cM interval at approximately 80 cM on chromosome 5HL. Our results will help identifying the role of the lab gene in relation to other spikelet fertility factors in barley.