The purpose of this study was to gather base-line life history data on the harbor porpoise. The information presented here describes only those aspects which would affect a census. During this two-year study conducted from two land sites in Glacier Bay National Park, Alaska, the authors investigated variability of distribution by season and habitat and behavioral changes which would affect a census by season and habitat. One study area, Sitakaday Narrows, had strong currents, and comparatively clear, deep waters. The other site, Adams Inlet, was a protected, turbid, shallow inlet. Density (porpoise/km2) within the study plots was highly variable by hour; in Sitakaday it also varied significantly by season: x̄ = 1.2 porpoise/km2 in July to x̄ = 5.9 porpoise/km2 in September. No such seasonal density changes were observed in Adams (x̄ = 1.2 porpoise/km2). Sightability of porpoise was affected by various behaviors which changed by season and location. Group size increased significantly in Sitakaday in fall and winter. The surfacing rate was greater in Adams than in Sitakaday: 2.5 surfacing per minute and 1.9 surfacings per minute, respectively. Frequency of a behavior called ‘staying-at-the-surface’, which greatly increased a porpoise’s sightability, varied for season and location. A splashing behavior we called ‘pop-splashing’ increased significamly in Sitakaday in fall and winter and most often occurred in association with the approach of vessels. Pop-splashing resulted in harbor porpoise often being misidentified as Dall’s porpoise.