The catalase multigene family in Arabidopsis includes three genes encoding individual subunits that associate to form at least six isozymes that are readily resolved by nondenaturing gel electrophoresis. CAT1 and CAT3 map to chromosome 1, and CAT2 maps to chromosome 4. The nucleotide sequences of the three coding regions are 70 to 72% identical. The amino acid sequences of the three catalase subunits are 75 to 84% identical and 87 to 94% similar, considering conservative substitutions. Both the individual isozymes and the individual subunit mRNAs show distinct patterns of spatial (organ-specific) expression. Six isozymes are detected in flowers and leaves and two are seen in roots. Similarly, mRNA abundance of the three genes varies among organs. All three mRNAs are highly expressed in bolts, and CAT2 and CAT3 are highly expressed in leaves.