Pectinaria (Lagis) koreni (Malmgren) is an abundant, deposit-feeding, infaunal inhabitant of shallow-water marine environments in northern Europe. Laboratory experiments were performed to quantify the polychaete's sediment processing in 2 distinct sediments, 1 fine-grained and high in combustibles, the other coarse-grained and low in combustibles. Gut passage time and time to pseudodefecation were predictable only in coarse-grained sediment. In both sediments, reworking rate increased with worm size and over time, although temporal patterns differed in the 2 sediments. The ratio of pseudodefecated sediment to defecated sediment did not differ significantly over time in either sediment, but the ratio was greater in the fine-grained sediment during the first measurement period. Another thrust of this study was to investigate possible size-selective ingestion of sediment particles by P. koreni. Results corroborated previous workers' findings that pectinariids selectively ingest large particles, a behavior at variance with current perceptions of optimal foraging by deposit feeders. These experiments were supplemented by natural history observations of the worm's infaunal behavior and particle-handling techniques.