Shipboard surveys were conducted in the summer of 1991 to estimate the abundance of harbor porpoises, Phocoena phocoena, in the northern Gulf of Maine and lower Bay of Fundy. The best estimate is 37,500 (95% CI: 26,600 to 86,400). The estimate is based on the independent team sighting procedure from one ship, which allowed the estimation of g(0): 0.72 (SE=0.06). The abundance estimate presented is substantially higher than previous estimates for the area. This is due to the more extensive areal coverage and the inclusion of g(0) into the estimate. While the statistical precision that was reported here included some sources of uncertainty, others remain to be addressed. The most important of these are the potential downward bias due to animals avoiding the vessel, the highly aggregated spatial distribution of porpoises and the largely unknown effects of heterogeneity of observers, platforms and environmental conditions.