The nonheme chloroperoxidases from Pseudomonas pyrrocinia and Streptomyces aureofaciens Tii24 are the only haloperoxidases with substrate specificity known until now. The chlorinating ability of these two bacterial nonheme haloperoxidases was investigated in detail. In contrast to eukaryotic chloroperoxidases, these two enzymes did not chlorinate monochlorodimedone, although this substrate was brominated by these enzymes. The antifungal antibiotic pyrrolnitrin [3-chloro-4-(3-chloro-2-nitrophenyl)pyrrole], however, was chlorinated by the two enzymes, yielding identical products. A mono-and a dichlorinated pyrrolnitrin derivative were isolated. In both cases chlorination occurred in the pyrrole ring. Furthermore, dioxypyrrolnitrin was formed by chemical oxidation in the presence of higher concentrations of hydrogen peroxide. The formation of this product could be suppressed by choosing suitable conditions. The pH optima for the chlorination of pyrrolnitrin, as well as the reaction velocities, were different for the two haloperoxidases. The chlorination of pyrrolnitrin to the same products by both enzymes suggests that the active sites of the two chloroperoxidases are very similar, although pyrrolnitrin is only a natural substrate for the Pseudomonas enzyme.