This article reports evidence for biologically mediated precipitation of dolomite in a fresh water spring in the south-east of Spain. Build-ups in the spring consisted of calcite, minor dolomite, barite and gypsum crystals formed by the mineralisation of microbial biofilms and bryophytes. A detailed microscopic study, coupled with geochemical analysis of the dolomite-precipitating biofilms, showed that dolomite crystals occurred in association with living coccoid micro-organisms, providing strong evidence that micro-organisms play a fundamental role in the precipitation of this mineral. Many of the calcite crystals observed were associated with extracellular polymeric substances. Moreover, microbial involvement in the precipitation of calcite was further supported by the presence of living cyanobacteria within the calcite crystals. Two different types of build-up were found in the spring: soft spongy moss tufa and laminated crusts. Two types of crusts were identified as follows: simple pink crusts and laminated crusts, the latter forming in both sub-aquatic and sub-aerial environments. Sub-aquatic accumulation of simple detached pink crust fragments occurred sporadically. Both tufa and the carbonate crusts hosted cyanobacterial-dominated biofilms with different bacteria and diatoms and their extracellular polymeric substances. The δ13C values of calcite showed some biogenic involvement in the origin of this mineral. The δ18O values were similar in the tufa and carbonate crusts and corresponded to current climatological characteristics. The occurrence of different groups of minerals (carbonates and sulphates) in close spatial association with micro-organisms suggests the presence of different macro-environmental and micro-environmental conditions that facilitate mineral precipitation within biofilms. These findings extend the known range of aerobic microbial dolomites to include springs, and show that the precipitation of both dolomite and calcite carbonates occurs while organisms are alive.