A new species of monostiliferoidean nemertean, Tetrastemmafozensis, from the mantle cavity of the bivalve mollusc Scrobicularia plana, in the Ría de Foz, north‐western Spain, is described and illustrated. Other nemerteans from the mantle cavity of Scrobicularia collected in Poole Harbour, southern England, are provisionally identified as conspecific with the Ria de Foz material, but show certain anatomical differences whose taxonomic significance cannot at present be assessed. Copyright © 1991, Wiley Blackwell. All rights reserved