Abstract: On the quantitative study of fungi in forests and bogs. The literature on the quantitative study of fungi in forests and bogs is discussed. An investigation of mushroom productivity in the area of Oulu is reported. All fruiting bodies of larger fungi were collected on ten occasions during late summer and autumn in 1970, in each of three sample areas in spruce forest, pine heath and pine bog. The specimens were sorted, counted and weighed fresh and after drying, and the fungus biomass per hectare was calculated. The yield was greater, and the productive season began somewhat earlier in spruce forest than in pine heath, but the season lasted later in the autumn in pine heath. The yield of the bog was scant and uniform. On the whole the yield was slightly above average.