The effect of gamma-radiation on the survival of microorganisms has been quantified for the natural population of two types of peat. Data for several microbial types have been separately determined by regular plating and by indirect statistical probability estimates including, a wholly enclosed 'inverted-bottle' technique for higher dose levels to exclude any possibility of post-treatment contamination. The most persistent microorganisms at intermediate dosage (2.5-3.5 Mrad) were commonly a micrococcus (which closely resembled Micrococcus radiodurans) arthrobacter-like rods, myxobacteria and amoeboid forms. The persistent organisms all survived because of high resistance to ..gamma..-irradiation, not because of high initial numbers. The most numerous true bacteria (including spore-formers), actinomycetes, filamentous fungi and yeasts were all readily destroyed. Although the safety margin with the commercially recommended dose of 5 Mrad is low for some of the more resistant organisms, no change is justified at this stage since the organisms most likely to survive such a dose do not seem to seriously affect the subsequent growth and survival of rhizobia. Moreover there would be some risk of radiation-induced peat toxicity if higher doses were applied and some post-irradiation contamination will be difficult to avoid in commercial production.