Detectability of a 100‐ms 1000‐Hz sinusoid was measured in quiet and in broadband noise having a spectral level of 20 dB SPL. Intensity discrimination of an in‐phase increment was studied for standards that were −9, −6, −3, 0, 3, 6, 9, 12, and 18 dB relative to threshold. When discrimination thresholds were plotted in terms of signal amplitude versus standard level, all subjects showed nonmonotonic functions (negative masking). Smallest thresholds occurred when the standard was just detectable. These discrimination thresholds were about 10 dB lower than detection thresholds. Psychometric functions were measured for simple detection (no standard) and for intensity discrimination with a standard at threshold. For intensity discrimination,d′ was proportional to signal amplitude. For simple detection,d′ was proportional to signal amplitude to the fourth power. The results in noise agreed well with those in quiet, although there was a tendency to obtain more “negative masking” in quiet and psychometric functions for detection were slightly steeper in quiet. [Work supported by NSF.]