Above-ground and below-ground biomass of the marine angiosperm Zostera marina (eelgrass) in central Barnegat Bay were determined using 15 cm diameter cores which were removed and cut into 5 cm sections. The ashfree dry weights of samples were determined. There was a weak, but statistically significant relationship between above-ground and below-ground biomass at two of the three sites sampled. Z. marina in the third site suffered a massive die-off in early August 1998. This die-off was first evident as a catastrophic reduction of above-ground biomass followed by a slower decrease in below-ground biomass. The lag in the decrease in below-ground biomass may be due to the slow rate of decomposition of the root mass. Below-ground biomass was limited to the top 5 cm of sediment at two sites (Forked River and Sands Point) but roots and even some rhizomes extended into the 5 to 10 cm depths of the soft, sandy mud sediments at the third site (Sedge Island). At all sites and on all sampling dates, concentrations of nitrate, nitrite, and ammonia were found to be below the levels of detection (nitrates < 0.1mg/l, nitrites < 0.15mg/l, and ammonia < 0.6 mg/l).