We have screened about 1.4 million colonies of Escherichia coli K-12 for their ability to grow on nutrient agar at 30 degrees and 41 degrees. Among the 2266 temperature-sensitive mutants found, 110 were defective in DNA synthesis but not in protein synthesis at 41 degrees. Three of these dna mutations mapped at two previously undescribed loci on the E. coli genetic map and may represent new genes involved in DNA replication in E. coli. The mutant isolation was aided by novel automatic machinery that inoculated agar-filled petri dishes with mutagenized E. coli cells laid down in square arrays of evenly spaced rows and columns on the agar. Time-lapse photographs taken before and after a temperature shift were used to find colonies of temperature-sensitive mutants. These mutations were mapped by interrupted conjugation and viral cotransduction methods, and the mutants were divided into three classes according to the kinetics of DNA synthesis at the restricted temperature. Some of the mutants exhibited mutator activity at partially restrictive temperatures. It is argued that some genes involved in DNA synthesis remain to be discovered.