In the brown alga Ascophyllum nodosum (L) Le Jolis, a common species on sheltered Northern temperate rocky shores, gametes are produced in receptacles that emerge from small depressions (lateral pits) along the branched frond. These lateral pits are also the preferred settling site for the obligate epiphyte Polysiphonia lanosa (L) Tandy. Therefore, epiphytism can be expected to interfere with host reproductive output. The present study investigated the potential impact of the epiphyte on A. nodosum in two series of laboratory experiments that measured: (i) the direct shading of the host plant underneath an epiphyte canopy; and (ii) the development of receptacles in clean and epiphytised A. nodosum segments (excised from individual fronds) over a 6 month period. These experiments showed that light reaching emerged fronds underneath a dense epiphyte cover was reduced by 40%, and this was independent of the degree of desiccation the epiphyte experienced. Concurrently, in the growth study with epiphytised A. nodosum segments (segments with one clean and one epiphytised lateral pit) total receptacle biomass per epiphytised fragment was significantly reduced compared with clean segments (0.52 g and 1.25 g per gram of frond segment, respectively), although this effect was only significant in A. nodosum from sheltered shores. However, expressed as biomass per lateral pit, receptacle biomass in the remaining clean lateral pits in epiphytised segments was significantly increased in segments from both shores, demonstrating that A. nodosum can at least partially compensate for the loss of production resulting from epiphytism.