To date, there has not been a single source to which the investigator can refer for guidance on the techniques of microbial ecology that have been successfully adapted and tested in sedimentary systems. STP 673, Methodology for Biomass Determinations and Microbial Activities in Sediments, fills this gap. It presents new data that concentrate on how to apply to sediments the newly developing technique of microbial ecology.. It is hoped that ASTM's STP 673 will not only provide a central source for information on the state of the art in sedimentary microbiology, but that these procedures will serve as references for future testing, modification and improvement.