The Chlamydomonas reinhardtii gene ARG7 encoding the enzyme argininosuccinate lyase is a popular marker for the nuclear transformation of this alga. We present the complete nucleotide sequence of ARG7. The gene spans 7 kilobases and is split by 12 introns containing highly repetitive microsatellite elements. Molecular analysis of eight different arg7 mutants indicates that none of the mutations is the result of gross rearrangements at the ARG7 locus. We describe our analysis of factors influencing transformation efficiency and the construction of a cosmid library of the Chlamydomonas genome in a customised vector carrying the ARG7 marker. This library should greatly facilitate the cloning of Chlamydomonas genes by complementation.