.......Whether ammonia-oxidizing archaea in soil can assimilate CO2 remains uncertain........... We were able to demonstrate active CO2 fixation and carbon assimilation coupled to ammonia oxidation by archaea in an agricultural soil using an RNA-SIP approach. Archaeal amoA transcripts were labeled consistently during incubation of soil microcosms with 13CO2 and fertilization with either 15 μg or 100 μg (NH4)2SO4-N·g−1 d.w.s. Controls without fertilization and with 12CO2 did not show any labeling, confirming that CO2 fixation was coupled to ammonia oxidation and that labeling resulted from true label incorporation. Analyses of archaeal amoA transcripts and genes from gradient fractions of the microcosms with the lower level of fertilization (15 μg N·g−1 d.w.s.) revealed clear differences and dynamic changes of archaeal ammonia oxidizers representing amoA clusters that differ in activity and in assimilation of carbon..........