Various fungi and myxomycetes have been tested for their synthesis and hydrolytic degradation of β-poly(L-malate) (PMLA), a highly anionic polyester of L-malate-malate. The PMLA content of the culture medium, and also of some cell extracts, has been assayed by L-malate-malate dehydrogenase (EC 1.1.1.83)-catalysed reduction of NAD+ after the alkaline hydrolysis of the polymer. All eight myxomycete isolates were producers. Eight producers were Mitosporic fungi but only one Ascomyete was productive. Aureobasidium pullulans contained PMLA in yeast-like cells but not in hyphae. The polymer was bound to membrane fractions from where it was slowly released during preparation for analysis. Only 12 of the 237 fungi and myxomycetes showed PMLA-hydrolase activity; and two of these were producers of PMLA. Fungi with hydrolase activity were mostly different from those releasing L-malate. The Physarum polycephalum-type unbranched polymer of high molecular mass was not recognized in fungi. Fungi, typically A. pullulans, produced polymer of low molecular mass which seemed to be covalently bound to polysaccharide.