Spore germination and growth of thermoactinomycetes were observed by epifluorescence microscopy. Each of the principal stages in germination was recognized and found to correspond to changes in phase-contrast appearance commonly monitored during endospore germination. The effects of novobiocin and nalidixic acid on germination of Thermoactinomyces vulgaris and T. thalpophilus were studied by using epifluorescence microscopy. Outgrowth but not initiation was inhibited by 100 μg of nalidixic acid per ml and 50 μg of novobiocin per ml. When each of the compounds, at various concentrations, was incorporated into a medium for thermoactinomycete recovery, the antibiotics were found to inhibit colony development. Samples of water and sediment incubated in a growth medium containing novobiocin and selective for thermoactinomycete species were examined by epifluorescence microscopy for total numbers of outgrown spores. Direct viable counts of outgrown spores indicated that the standard plate count enumerated less than 10% of the viable population of thermoactinomycetes.