Lacustrine stromatolites occur in the Pliocene Ridge Route Formation which was deposited in the tectonically active Ridge Basin, California. The stromatolites developed as hard, resistant bulbous and commonly coalesced structures as much as 0.5 m in diameter, and were subject to wave agitation and periods of terrigenous influx and subaerial exposure. The stromatolites formed on hard, irregular substrates within pelitic lithotopes, which occur in fining-upward sedimentary sequences. Stromatolitic growth was favored in relatively quiet-water, nearshore environments, whereas a freshwater molluscan assemblage coexisted in adjacent, higher-energy, sandy environments. Stromatolitic microstructure consists of irregular concentric layers of radially fibrous, low magnesium calcite separated by laminae of dark, organic material containing peloids and coccoid and filamentous algae. Insoluble residue extracted from the stromatolites is very fine- to coarse-grained sand, which represents from 3 to 6 percent by weight. Oncolites, pisoliths, "high-energy" ooids and ostracods are commonly associated with and incorporated into stromatolitic laminae. Relatively widespread development of stromatolites in nearshore environments may indicate periods of tectonic quiescence and/or low rates of detrital influx into the Ridge Basin.