When converting analog signals into the digital domain, steep low-pass filters are required to avoid aliasing. Aliasing is the 'pollution' in the frequency range of interest created by frequencies higher than the Nyquist frequency (half the sampling rate). Good anti-aliasing filters are very expensive and, probably, not strictly necessary when recording music and speech for human listening. On the other hand, when recording/analyzing sounds with unknown frequency con' tents, or with components, including harmonics and unpredictable noises, extending beyond the Nyquist frequency, steep filters are required to avoid aliasing. As DATS are widely used to record animal voices, we made some tests on their anti-aliasing filters to evaluate artefacts possibly present on the recording. Results were compared with those coming from some PC sound boards and from anti-aliasing filters used with signal acquisition instrumentation. All the recorders and sound boards we tested showed different degrees of aliasing occurring in the upper range of their bandwidth. Even if its effect might be considered unnoticeable to human listening, aliasing must be taken into consideration when analyzing recordings or when performing playback experiments.