The cosmopolitan sponge Dysidea fragilis has been shown to produce several different sesquiterpenes and the nature of these natural products shows a marked geographic variation. For instance the sponge collected from Hawaii yielded the bicyclic sesquiterpenes upial and the nakafurans 8 and 9. Extraction of D. fragilis from Brittany, on the other hand, gave a series of sesquiterpenes based on the monocyclic sesquiterpene penlanfuran. In surprising contrast to these reports we have found that D. fragilis (Montagu, 1818) collected in Fiji contains no sesquiterpenes but a new cytotoxic azacyclopropene carboxylic acid ester, dysidazirine (1). This represents the first example of this ring strained class of heterocycles from a marine source and, to our knowledge, only the second reported naturally occurring 2H-azirine.