A new CMS system designated as 'msH1' has been reported in bread wheat using the cytoplasm of H. chilense. While testing this system in different wheat backgrounds, a highly fertile line with chromosome number 42 plus an extra acrocentric chromosome was obtained. The extra chromosome did not pair with any wheat chromosome at meiosis, and progeny from this line which lack the acrocentric chromosome showed pollen abortion and male sterility. In order to establish the origin of this chromosome, FISH using H. chilense genomic DNA as probe was used and showed that it had originated from H. chilense chromosome(s). The novel chromosome did not possess sequences similar to wheat rDNA; however, the probe pSc119.2 from S. cereale containing the 120 bp family was found to occur at the end of its long arm. Data obtained from FISH and EST molecular markers confirm that the long arm of the acrocentric chromosome is indeed, the short arm of chromosome 1H(ch) from H. chilense. We suggest that the novel chromosome originated from a deletion of the distal part of the long arm of chromosome 1H(ch). Neither the 1H(ch)S short arm, nor the whole chromosome 1H(ch) restores pollen fertility of the alloplasmic wheat. Therefore, the restorer gene on the acrocentric chromosome must be located on the retained segment from the hypothetical 1H(ch)L, while some pollen fertility inhibitor could be present on the deleted 1H(ch)L distal segment. Disomic addition of the acrocentric chromosome was obtained and this line resulted fully stable and fertile.