Initiation and termination of chromosome replication in an Escherichia coli auxotroph subjected to amino acid starvation were examined by following the incorporation of [3H]thymidine into the EcoRI restriction fragments of the chromosome. The pattern of incorporation observed upon restoration of the amino acid showed that starvation blocks the process of initiation prior to deoxyribonucleic acid synthesis within any significant portion of the EcoRI fragment which contains the origin of replication, oriC. In this experiment, no incorporation of [3H]thymidine into EcoRI fragments from the terminus of replication was observed, nor was it found when a dnaC initiation mutant was used to prevent incorporation at the origin which might have obscured labeling of terminus fragments. Thus amino acid starvation does not appear to block replication forks shortly before termination of replication. Attempted synchronization of replication initiation by including a period of thymine starvation subsequent to the amino acid starvation led to simultaneous incorporation of [3H]-thymidine into all EcoRI fragments within the 240-kilobase region that surrounds oriC. It is shown that the thymine starvation step allowed initiation and a variable, but limited, amount of replication to occur.