Recent and older (early Holocene to Pleistocene) hot-spring travertine carbonates from central Italy have two distinctive macrofabrics, crystalline crusts and shrubs. Crystalline crusts are laminated slope deposits that formed abiotically following CO 2 degassing from spring water. The formation of shrub travertine--irregular, dendritic precipitates from pool environments--is controversial and has been attributed to both abiotic and microbial processes. Oxygen isotope variation in our travertines can be explained by abiotic processes, mainly CO 2 degassing. In contrast, our carbon isotope data cannot be wholly explained by abiotic CO 2 degassing invoked in earlier studies. Because photosynthesis is known to preferentially remove 12 C, leaving ambient waters enriched in 13 C, this fractionation should be recorded in delta 13 C values of microbially influenced travertine. Our shrub carbon isotope values are between 0.5 per thousand and 6.0 per thousand larger than values for associated abiotic precipitates, and this difference is probably caused by microbial activity. These isotope data support previous visual evidence that some shrub fabrics are microbially influenced.