SummaryTo obtain further evidence of the contribution of autochthonous organic matter to lake sediments, the benzene‐methanol extract of the particulate matter remaining after microbial attack upon an alga, Ceratium hirundinella, during laboratory simulation of natural decomposition processes, has been separated into its components. Comparison of each component with corresponding material isolated from the fresh alga facilitates the recognition of unchanged substances and microbial products in the decomposed sample. Certain common features in the occurrence and distribution of n‐alkanes, alkenes, n‐ and branched/cyclic alkanoic acids, alkenoic and hydroxy acids in degraded algal organic matter and in sediments of productive lakes are believed to indicate a contribution of autochthonous material to the sed