Aim: To study the microbiology of intensive, in-vessel biodegradation of a mixture of sewage sludge and vegetable food waste. Methods and Results: The biodegradation was performed in a closed reactor with the addition of a starter culture of Bacillus thermoamylovorans SW25 under conditions of controlled aeration, stirring, pH and temperature (60degreesC). The content of viable bacterial cells, determined by flow cytometry, increased from 5 x 10(8) g(-1) of dry matter to 61 x 10(8) g(-1) for 6 days of the process and then dropped to the initial value at the end of the process. The reductions of organic matter, 16S rRNA of methanogens and coenzyme F-420 fluorescence during 10 days of the treatment were 67, 54 and 87% of the initial values, respectively. The biodegradability of the organic matter decreased during the 10 days of the treatment from 3.8 to 1.3 mg CO2 g(-1) of organic matter per day. The treatment of sewage sludge and food waste at 60degreesC did not remove enterobacteria, which are the agents of intestinal infections, from the material. The percentage of viable enterobacterial cells, determined by fluorescent in situ hybridization (FISH) with Enterobacteriaceae-specific oligonucleotide probe and flow cytometry, varied from 1 to 14% of the viable bacterial cells. Conclusions: The mixture of sewage sludge and food waste can be degraded by the aerobic thermophilic bacteria; the starter culture of Bacillus thermoamylovorans SW25 can be used to perform this process; and enterobacteria can survive under treatment of sewage sludge and food waste at 60degreesC for 13 days. Significance and Impact of the Study: The results show that FISH with an oligonucleotide probe can be used to study not only the growth but also the degradation of biomass. Obtained results could be used to design the bioconversion of sewage sludge and food waste into organic fertilizer.