Population density, biomass, seasonal growth and gonad index of the heart urchin Echinocardium cordatum were monitored in muddy sediments of the Seto Inland Sea, Japan, from July 1998 to July 2000. Rates of sediment ingestion by the urchin were also measured. Juveniles settled in winter and their growth in the first year was rapid. Individual somatic mass in terms of ash-free dry weight (AFDW) increased from about 0·1 to 40 mg while the test length increased from about 2 to 20 mm. At the end of the second and third year (early December), somatic mass averaged 90 and 120 mg AFDW (test length=30 and 33 mm), respectively. The gonads of both year-classes matured during this period and spawning followed within two months. Individual E. cordatum (test length=29–34 mm) ingested sediments at rates of 14–43 mg wet weight per hour. Population density (28 m−2) and AFDW biomass (2·8 g m−2) of E. cordatum (test length [ges ]15 mm) were high at the start of monitoring (July 1998), remained relatively constant through the next spring and rapidly decreased to <2 m−2 and <0·2 g AFDW m−2, respectively, by December 1999. When abundant, E. cordatum comprised about 60% of the macrobenthic biomass in the study area and its annual production in AFDW reached about 1·5 g m−2 y−1, suggesting substantial contributions of the urchin in material cycling through the sediments.