An acrotrisomic plant was identified in the progeny of a telotrisomic for 1S. The acrocentric chromosome had a complete short arm (5S) and 40% of the proximal segment of the long arm (5L). Morphology of the acrotrisomic 5S5L was similar to the primary trisomic (triplo 5) and triplo 5L. At meiosis the acrocentric 5S5L either paired with its normal homologues forming a trivalent or remained as a univalent. Seed fertility was high. Transmission of the acrocentric was 37.6% through the female and 9% through the male. Genetic tests showed that fs2 and g were located in this 40% proximal segment of the 5L. Gene f3 showed a trisomic ratio with acrotrisomic 5S5L, but its arm location is not known. Two genes, f7 and trd, were located on the 60% distal segment of the 5L. The segregation ratio with gene int-a1 was also disomic but it could not be assigned to the 60% distal segment because its location on chromosome 5 is questionable at this time. This experiment demonstrates the usefulness of acrotrisomics in physical gene mapping by locating genes on a specific segment of the chromosome arm.Key words: acrotrisomic, barley, acrocentric, trisomic, telotrisomic.