The threshold for detection of a narrow-band noise signal was determined in the presence of a masker consisting of two synchronously gated 20-Hz-wide bands of noise with a spectrum level of 65 dB which were comodulated. The maskers were spaced by (Delta)F Hz above and below the signal frequency (1500 Hz). A low-pass noise was used to mask any combination products. Three conditions were tested: signal and masker envelopes correlated (C), signal and masker envelopes uncorrelated (U), or sinusoidal signal of the same overall level (S). (Delta)F ranged from 200 to 1400 Hz. Masked thresholds for the U and S conditions were essentially identical for all (Delta)F. Thresholds were higher in the C condition, thus showing a comodulation detection difference (CDD). The CDD was greatest for (Delta)F between 400 and 600 Hz. A second set of conditions was used in which either one, both, or none of the masking bands was comodulated with the signal. This was done for three different masker spectrum levels at a (Delta)F of 600 Hz. The results suggest that the CDD seen in the first experiment was mainly due to the upward spread of masking from the lower band and not to an across-channel grouping effect.