Le Zitelle hot springs contain high concentrations of dissolved Ca and CO2 and precipitate copious quantities of aragonite and calcite as travertine. A study of precipitation rates in the upper section of the stream was undertaken using marble tablets. This showed that: (1) precipitation rate increased with distance from the spring over the section studied; (2) the rate was positively correlated with flow, agreeing with previous experimental and theoretical studies; (3) precipitation rates during the day and night did not differ statistically. The last finding demonstrated a negligible effect of photosynthesis on travertine formation and was supported by further precipitation rate studies undertaken in clear and blackened tubes. This conflicts with previous work that suggested an important role for microbial photosynthesis in travertine deposition at Le Zitelle. The microbial flora consisted mainly of cyanobacteria (Spirulina) and photosynthetic bacteria, with smaller quantities of diatoms. The biomass in most of the travertine was extremely low and often endolithic. Carbon fixation rates were also low. A mass-balance calculation of carbon flow showed that of 100 units of dissolved carbon entering the stream section investigated, 51 units were lost to the atmosphere as CO2, 0.9 units were deposited as travertine, 0.09 units were fixed by photosynthesis and 48 units remained in the stream water. We conclude that most of the Zitelle travertine is formed abiotically, casting doubt on the supposed biotic origin of travertine at this and other Italian sites.