A tonal masking experiment of the sort pioneered by Wegel and Lane was conducted with both monotic and dichotic stimulation. The forms of the masking patterns under these two conditions of listening were markedly different. The monotic data were similar to the classical data—there was a local minimum in masking at the masker frequency, gradually more masking at adjacent frequencies, and then a gradual, and asymmetric, decline in masking at frequencies more distant from the masker. For the dichotic condition, however, masking wasgreatestat the masker frequency, and it showed two local minima at about ±10 Hz from the masker before declining gradually for more distant frequencies. These two minima are interpreted as reflecting a kind of time constant in the binaural system, an interpretation that receives support from research on binaural beats. Across signal frequencies, the masking‐level differences (MLDs) ranged from about 1 to 11 dB.Masker:400 Hz, 70 dB SPL, continuously present.Signal:100‐msec duration, 10‐msec rise‐decay time.Method:Two‐interval forced choice with feedback.Listening conditions:Nm‐Sm and N0‐Sπ.