The physical and biogenic sedimentary structures of Mugu Lagoon, a shallow coastal lagoon in Southern California, were investigated in an effort to define suites of sedimentary structures and use them to divide the lagoon into sedimentary subenvironments. These data, coupled with grain size information, were used to prepare a depositional environment map of Mugu Lagoon. Seventy-three "can-cores" were taken along traverses in the lagoon. X-ray radiography and epoxy peel techniques were used to analyze the internal structures of the sediments. Twelve types of physical and six types of biogenic sedimentary structures were recognized, and the amount of bioturbation was estimated. On the basis of sedimentary structures the lagoonal system was divided into ten major subenvironments, which are: barrier beach, tidal delta, tidal channel, tidal creek, subtidal pond, barren zone or tidal flat, lower marsh, upper marsh, salt pan, and ancient sand ridge. Minor subenvironments , developed only locally within the lagoon, are: silt pellet dunes, tidal creek banks, and mussel beds. The subenvironments are characterized by different associations of physical sedimentary structures and the degree of biogenic reworking. A relationship between current strengths, sedimentation rates, and the amount of bioturbation exists. In subenvironments with rapid sedimentation rates and high energy regimes (e.g., barrier beach, tidal delta, tidal channel) physical structures predominate. In those areas with slow sedimentation rates and low energy regimes (e.g., subtidal ponds, upper marsh) biogenic structures are most common. Intermediate areas, such as the barren zone, are characterized by physical and biogenic structures in subequal amounts. The subenvironments defined by distinctive associations of sedimentary structures correspond extremely well with depositional environments recognized in Mugu Lagoon by other methods (Warme, 1966; 1971), showing that these depositional environments can be useful in interpreting similar modern analogs or ancient lagoonal systems.