ABSTRACT: We examined the natural variation of nitrification potentials (NPs) of surface sediments and macrofaunal tubes and burrow walls in relation to sediment NH₄⁺ level, season, and macrofaunal species. NP (the ability of a unit ofsediment to oxidize NH₄⁺ when NH₄⁺ and O₂ are not limiting) is an index of the abundance and activity of nitrifying bacteria which we measured in slurries with the chlorate block technique (nmolNO₂^--N produced g^-1 dry weight sediment h^-1). The NP of the tubes of the polychaete Loimiamedusa was positively related to sediment NH₄⁺ (KCl-extractable) concentration at 3sites where tubes were collected in June 1990 (Spearman rank correlation coefficient r_S = 0.90, p = 0.03), as was the NP of surface (0 to 1 cm) sediment (r² = 0.92, p = 0.002). The degree to which tube NP exceeded the NP of surfacesediment was, however, negatively associated with sediment NH₄⁺ (r_S = -0.84, p = 0.05). Tube NP of L. medusa did not vary significantly with date (February, April, and June 1990). Tubes or burrow walls ofMacomabalthica (bivalve), Leptocheirusplumulosus (amphipod), and the polychaetes Macroclymene zonalis, Pectinariagouldii, L.medusa, and Diopatracuprea had NPs significantlygreater (2 to 20 times) than that of adjacent sediment from the same depth interval, indicating that these species stimulated nitrification. Except for burrows of M. balthica, the NPs of these structures were significantly (p &LT= 0.05)greater (1.5 to 61 times) than that of surface sediment. The duration of macrofaunal irrigation activity, but not irrigation rate, was positively associated (r_S = 0.72, p = 0.01) with the enhancement of NP in tubes and burrow walls relative tosurface sediment. These findings indicate that macrofaunal tubes and burrows tend to be sites of enhanced NP and that this enhancement varies among species due to variations in irrigation behavior. The NP of macrofaunal structures also varies among sitesin relation to sediment NH₄⁺ concentrations.