ABSTRACT: We investigated whether intraspecific competition for food and/or other possible density-dependent interactions occurred within and between 2 age classes of the deposit-feeding amphipod Monoporeia affinis. A 2-factor laboratory experimentwas conducted, where juveniles (0+) and adults (1+) were kept in experimental jars for 2 mo at densities of 2000, 4000 and 8000 ind. m^-2 , which corresponds to natural abundances in the Baltic Sea.The amphipods were fed on amixture of naturally occurring algae at 3 food levels: zero, medium (representing a diatom spring-bloom) and high (representing a 2 x diatom spring-bloom). The level of food added to the experimental jars determined whether foodcompetition occurred, both within and between the 2 age classes. Growth of juveniles was not density-dependent, at any food level, in the absence of adults, but clearly dependent on presence or absence of fresh algal material. In presence of high adultdensities, the growth of juveniles was significantly reduced at food levels that corresponded to a single and twice a diatom spring-bloom, while such reduction was not found in jars without added algae. Growth of adults was both density- andfood-dependent. Juveniles did not affect the survival and growth of adults. As high densities of amphipods in jars without added food affected neither growth nor mortality, we conclude that crowding effects such as predation or directly damaginginterference are not important regulating mechanisms. The results indicate that exploitative intraspecific competition for food occurs both between and within year classes, but depends on the resource levels and is likely to be most intense under a regimeof low to medium input of phytodetritus. The experiment lends support to the hypothesis that intraspecific competition for food acts as an important process causing fluctuations in the Baltic Sea populations of M. affinis.