Analysis of NADH dehydrogenase isozymes in hexaploid wheat (Triticum aestivum) by flat-bed isoelectric focusing revealed a genetic system involving four sets of Ndh loci. The Ndh-A1 locus, previously identified on chromosome arm 4AL, was confirmed. Homoeoloci Ndh-R1 in rye, Ndh-H1 in barley, Ndh-Hch1 in Hordeum chilense, Ndh-Sl1 in Aegilops sharonensis, and Ndh-V1 in Dasypyrum villosum were also identified. The Ndh-2 loci, encoding monomeric isozymes, were found in the short arms of homoeologous group 7 chromosomes. Homoeoloci Ndh-A2, Ndh-D2, and Ndh-R2 are described. The Ndh-3 set, encoding multimeric isozymes, were found on each of the homoeologous group 3 chromosome long arms. An Ndh-B3b variant and homoeoloci Ndh-R3 (on chromosome arm 6RL), Ndh-H3, and Ndh-Sl3 are described. The Ndh-4 set, encoding monomeric isozymes for which no varietal variation was found, were identified on the short arms of group 3 chromosomes. Ndh-A4, Ndh-B4, Ndh-H4, Ndh-R4, and Ndh-E4 (Agropyron elongatum) homoeoloci are described.Key words: NADH dehydrogenase, isozymes, wheat, Triticeae.