lake trout reproductive habitat was characterized in three small lakes, 250 km northwest of thunder bay, ontario, by measuring the physical characteristics of preferred spawning habitat including 1) depth, 2) substrate size, 3) interstitial space depth, 4) organic material abundance, 5) embeddedness, 6) particulate debris, and 7) permeability. principal components analysis indicated that periphyton, macrophyte, and particulate debris abundance all increased with shoal depth, while substrate size decreased. hydraulic permeability, indexed by the erosion of gypsum cylinders, was higher in coarser substrates. lake trout egg deposition density in egg traps averaged 70 eggs m'2, of which 45% were viable by late fall lake trout embryo survival and emergence in enclosures varied with fredle index, and was highest (75%) in cobble/rubble mixtures. fine sediment which was added to incubators in the fall was absent when the incubators were retrieved in the spring. at the single fine sediment dosage tested in this study (equivalent to a layer approximately 2.5 cm deep across the surface of each incubator), lake trout hatching success was not significantly affected. although experimental nutrient enrichment cp and n) of a spawning shoal increased periphyton biomass by 2.5 times over the summer, the effects of this on reproductive habitat are not known at present.