In this article we present estimates of data transmission error performance at data rates of 1200, 2000, 3600 and 4800 b/s on the Bell System Switched Telecommunications Network. The source data was collected as part of the 1969–70 Connection Survey conducted by Bell Laboratories. Results are based on measurements made on approximately 600 toll connections, dialed from 12 receiving to 92 transmitting sites in the United States and Canada. Standard Bell System Data‐Phone® data sets were used to transmit and receive the digital signals. Distributions of errors per call are given on a bit, burst and block basis. Information is also presented on the distribution of intervals between errors, the structure of error bursts, and the number of errors in blocks of various sizes. Some discussion is given on error causes observed for operation at 2000 b/s. Results of error rate measurements indicate that for operation at 1200 and 2000 b/s approximately 82 percent of the calls have error rates of 1 error in 105 bits or better, assuming an equal number of short‐, medium‐, and long‐haul calls. This represents a substantial improvement of performance in comparison with results of previous surveys. For each of the four data rates tested, 1000‐bit block error rates of less than one block error for every 102 data blocks transmitted are achieved on 80 percent of the calls. For operation at 2000 b/s, a major cause of errors is shown to be impulse noise.