On 7th September 2019, we observed an unusual juvenile marsh tern Chlidonias at Rochefort sewage treatment plant, Charente-Maritime, western France (plate 242). The bird was with a group of c. 30 Black Terns Chlidonias niger and one Whiskered Tern C. hybrida. It was closest in appearance to Black Tern but showed a number of features at odds with that species, such as a very small, diffuse pectoral spot, a pale grey rump fading to whitish at the sides and a rather shallow tail fork. The bird also seemed smaller and slimmer than the accompanying Black Terns. In these respects, it was closer to White-winged Black Tern C. leucopterus but the greyish centre to the rump, relatively extensive black on the head, colour of the feet, and subdued contrast between the mantle and wings were at odds with the typical appearance of this species.