SYNOPSIS.Twelve derivatives of 5‐nitrofuran were tested onEuglena grarzlis. All rendered the organism permanently apoplastidic and, at somewhat higher concentrations, killed. The furan analogues of 3 of these compounds had no effect on the chloroplast system and were less toxic than the nitrcfurans. Low concentrations of nitrofurantoin and nitroiuraldehyde inhibited formation of chlorophyll when etiolated cells irere illuminated.Exposure of euglena to low concentrations of these agents for about 2 generation times. followed by plating on drug‐free mediurn. resulted in a high proportion of bleached colonies. It is theretore concluded that the nitrofurans induce apotplastidy by causing permanent damage to the chloroplast system rather than by inhibiting its replication temporarily. Since one of the nitrofurans which was found to bleach euglena, NFT–3‐amino‐6[‐2‐(5‐nitro‐2‐furyl) vinyl]‐1,2,4‐triazine–is known to cause specific inhibition of DNA synthesis in bacteria, nitrofurans may perhaps bleach euglena through selective damage to chloroplast‐DNA or to the DNA‐synthesiz