Dimethyl sulfide (DMS) concentrations were measured over 2 years in 10 lakes situated in northwestern Ontario. Concentrations varied from 0.1 nmol L-1 to 100 nmol L-1 (geometric mean of 1.2 nmol L-1) during the ice-free season (April to November) of 1995 and 1996. Shallow (1-5 m depth and 1-5 ha area) and medium depth (5-10 m depth and 5-15 ha area) lakes exhibited higher surface water DMS concentrations than deeper and larger lakes (32 m depth and 20-56 ha area). During the fall in the medium size lakes (5-12 m depth and <15 ha area), DMS concentrations increased markedly by a factor of as much as 100 because of circulation of deeper, high-DMS water from the hypolimnion (lake turnover). The estimated seasonal mean fluxes of DMS from shallow and medium-depth lakes to the atmosphere ranged between 0.058 and 15 μmol S m-2 d-1 respectively. The shallow and medium depth lakes tend to release higher DMS than the large and deeper lakes. Extrapolation of flux estimates indicates that the lakes of the Canadian Boreal Shield emit 1.5 Gg yr-1 biogenic sulfur (DMS) to the atmosphere, and this is approximately 83% of the total annual biogenic sulfur (DMS) emissions from the Canadian Boreal Shield (lakes plus terrestrial). Compared to the annual anthropogenic emissions by five smelters in the boreal region, these emissions are small (0.08%). ¿ 1999 American Geophysical Union