The changes that protoanemonin, an antimicrobial agent of natural origin, brought about in the algaEuglena graciliswere studied with light and electron microscopy and with flow cytometry. The compound proved lethal for the alga at a dose of 5×10−5m. At the sublethal dose of 3.5×10−5m it caused: marked inhibition of growth; increase in the average cell volume; inhibition of cytokinesis and induction of coenocytic organisms; loss of the primary_articlegellum and stigma. Most nuclei were arrested in the G2/M phase of the cellular cycle. The chloroplasts were still well organized, but there was a conspicuous decrease in carotenoids and chlorophyllsaandbwith a corresponding increase in pheophytins. The multifarious alterations seen in Euglena are discussed on the basis of a possible interaction between this lactone and several enzymatic systems.