The assembled Physcomitrella patens genome has been released. This provides a unique opportunity to identify and characterize new genes, and to compare moss genes with other plant genes from previously released plant genomes. Aldehyde dehydrogenases (ALDHs) comprise a protein superfamily of NAD(P) -dependent enzymes [EC 1.2.1] found in bacteria, animals and green plants capable of oxidizing a variety of aromatic and aliphatic aldehydes. ALDHs are key enzymes of abiotic stress-tolerance in a variety of organisms. In bryophytes, ALDHs are postulated to play a role in vegetative desiccation-tolerance. Using recently available genomic DNA sequence data, we have identified members of the ALDH gene superfamily in the marine picoeukaryote Ostreococcus tauri, the unicellular green algae Chlamydomonas reinhardtii and the moss Physcomitrella patens. The Arabidopsis thaliana genome has been shown to contain 14 unique ALDH sequences encoding members of nine ALDH protein families. The O. tauri genome contains six unique ALDH sequences encoding members of six ALDH protein families. The C. reinhardtii genome contains eight unique ALDH sequences encoding members of seven ALDH protein families. The P. patens genome contains 20 unique ALDH sequences encoding members of ten ALDH protein families. These three genomes encode members of ten previously identified protein families (ALDH2, ALDH3, ALDH5, ALDH6, ALDH7, ALDH10, ALDH11, ALDH12, ALDH21 and ALDH22) and two novel protein families (ALDH23 and ALDH24). We analyzed a sequence alignment of ALDH deduced amino acid sequences among plants using neighbor-joining and maximum parsimony methods. These analyses demonstrate that 1) at least eight ALDH protein families, as represented in the O. tauri and/or C. reinhardtii genomes were present prior to the evolution of land plants and each of these families have been maintained throughout land plant evolution, 2) novel ALDH families of as yet unknown function have evolved in several lineages of land plants including the mosses and flowering plants and 3) the moss P. patens exhibits a highly expanded set of sequences in three families, ALDH3, ALDH5 and ALDH11.