The population of the nitrifying bacteria from four different historical buildings in the Federal Republic of Germany (DDR) was analyzed. To obtain the most abundant nitrifying organisms, the isolation procedure started from positive MPN test tubes in the highest dilution steps. 10 pure cultures of ammonia oxidizers and 50 of nitrite oxidizers were isolated and characterized. Among the ammonia oxidizing bacteria members of the genera <i>Nitrosovibrio</i>, * 2Nitrosomonas*1, and <i>Nitrosospira</i> were detected. From 10 pure cultures eight were identified as <i>Nitrosovibrio</i>, one as * 2Nitrosomonas*1 and another one as <i>Nitrosospira</i>. The * 2Nitrosovibrio*1 strains grew best at substance concentrations of 10-80 mM ammonia and showed a great tolerance to salinity. Their temperature optimum fell in the range of 25° C to 30° C. Even at temperatures of 5° C or 50° C a slow growth could be observed. The pH-optimum was 7.2-7.4. The strains were characterized morphologically by carboxysomes and intracytoplasmic membranes. All nitrite oxidizers belonged to the genus <i>Nitrobacter.</i> These strains grew lithoautotrophically and mixotrophically, but not herotrophically except for three strains. -- AATA