To judge from reports in the literature, thermophilic Actinomycetes have proved difficult to isolate. For isolations from mouldy hay we have developed a convenient method which promises to be useful in other contexts1. The method uses the fact that dry air removes spores of the Streptomycetaceae in preference to bacteria. Instead of suspending the spores in water and plating dilutions in the traditional manner, dry spores are suspended in air and impacted with the Andersen sampler2 directly on surface-dried medium. In our work, samples of hay are shaken in a small wind tunnel and the air drawn into an Andersen sampler loaded with Petri dishes which had been poured the day before with half-strength nutrient agar (agar content made up to 2 per cent) containing 0.5 mgm./ml. actidione to suppress growth of mould. A slit-sampler could be used as an alternative to the Andersen sampler, and instead of using a wind tunnel, hay or other material can be shaken in a box (pre-sterilized with propylene oxide if required) and a few minutes allowed for larger particles to settle before sampling the air.