A new transposable element, Tcr3, was identified in the unicellular green alga Chlamydomonas reinhardtii. The Tcr3 element contained imperfect terminal inverted repeat sequences of 56 bp and created a 2 bp target site duplication upon insertion. Insertion of Tcr3 into the 3′-untranslated region of the NIT8 gene, which is essential for nitrate assimilation, prevented expression of the gene. Excision of the Tcr3 element correlated with reversion of the mutant phenotype and left behind a 3 bp footprint. Tcr3 was found in all Chlamydomonas isolates tested and should prove to be useful for transposon-tagging experiments in Chlamydomonas.