A study of the variation in sea ice algal distribution and production was performed during June and early July 1999 in Young Sound, northeastern Greenland. Measurements were performed in situ by divers using an underwaterpulse-amplitude-modulated (PAM) fluorometer, oxygen microelectrodes and ¹⁴C incubations. In early June, Young Sound was covered by ~1.6 m of sea ice and ~30 cm of dry snow, which reflected ~75% of the incident light. These conditions resultedin low irradiance below the sea ice (<2 µmol photons m^-2 s^-1), low values of chlorophyll a (<1 µg l^-1 sea ice) and low primary productivity (<0.05 µgC l^-1 h^-1) in the bottom ice (0 to4 cm). The snow cover gradually disappeared during June, resulting in increased light penetration of up to 20 µmol photons m^-2 s^-1, chl a concentrations of up to 15 µg l^-1 (0.6 mgC m^-2), and primaryproduction rates of up to 0.2 µgC l^-1 h^-1 (0.2 mgC m^-2 d^-1) in the bottom ice. Light saturation curves of photosynthesis were obtained in situ on 3 occasions and showed adaptation of the sea ice algae tolow irradiance; E_k values ranged from 12 to 35 µmol photons m^-2 s^-1. Photosynthesis of sea ice algae resulted in supersaturated oxygen conditions (150 to 200%) at the sea ice-water interface as compared to ambientseawater. In situ investigations of irradiance, algal biomass and photosynthetic activity on different metric scales showed a heterogeneous distribution of sea ice algae. Statistical treatment of the data obtained revealed that algal activityfollowed the light regime below the sea ice (radii of patches were 50 to 100 m), whereas algal biomass patches were smaller (radii of patches were 5 to 10 m). Sea ice algal production in Young Sound was 2 to 3 mgC m^-2 during June and July 1999.Compared to other coastal fast-ice areas this rate is low. We suggest that the low biomass and productivity in Young Sound was caused by a combination of poor light conditions due to snow cover and freshwater drainage from melt ponds and river dischargeremoving and/or inhibiting the algae at the sea ice-water interface through physical disturbance and exposure to freshwater. The primary production of sea ice algae in Young Sound was <1% of the annual pelagic primary production.