Barley (Hordeum vulgare L.) carrying the recessive mutation absent lower laterals (als) exhibits few tillers and irregular inflorescence development. To gain an increased understanding of the genetic control of tillering in barley, we conducted morphological, genetic, and transcriptome analysis of the als mutant. Axillary buds for primary tillers, but not for secondary tillers, developed in als plants. Double mutant combinations of als with one low-tillering and four high-tillering mutants resulted in a tillering phenotype similar to als, indicating that als was epistatic to these tillering genes. However, double mutant combinations of als with another low-tillering mutant, intermedium-b, reduced tiller numbers, indicating there were at least two genetic pathways regulating tillering in barley. Next, we used simple sequence repeat markers to map the Als gene on the long arm of barley chromosome 3H, Bin 11. Finally, the Affymetrix Barley1 GeneChip was used to identify differentially accumulated transcripts in als compared to wild-type. Forty percent of the transcripts with twofold or greater accumulation in als tissues corresponded to stress and defense response genes. This finding suggested that a tillering pathway may modulate the stress response.