It has been previously reported that 3-month-old human infants are more susceptible to forward masking than are adults of the same species. The purpose of the present study was to determine whether susceptibility to forward masking declines between 3 and 6 months of age. Six-month-old infants were trained to respond when they heard a train of alternating bursts of a broadband noise and a 1-kHz tone, but not when they heard the noise bursts alone. The tone burst had 16-ms rise and fall times and no steady-state duration. The interval between noise offset and tone onset, Δt, was either 10, 25, or 200 ms; Δt was fixed for each infant. The onset to onset interval between noise bursts was constant at 350 ms + Δt on all trials. The level of the noise was 65 dB SPL; its duration was 100 ms. The level of the tone was manipulated to define masked threshold. Six-month-olds demonstrated about 10 dB more forward masking than adults and about 10 dB less forward masking than 3-month-olds for short Δt, and about the same amount of masking as 3-month-olds for 200 ms Δt. [Work supported by NIH.]