For neurons in the auditory midbrain of the grass frog the use of a combined spectro-temporal characterization has been evaluated against the separate characterizations of frequency-sensitivity and temporal response properties. By factoring the joint density function of stimulus intensity, I (f, t), preceding a spike, into two marginal density functions I1(f) and I2(t) one may under the assumption of statistical independence reconstruct the joint density by multiplication: I1(f).I2(t). The reconstructed I(f, t) is compared to the original I(f, t) for 83 neurons: in 23% thereof the I(f, t) appeared to be vastly different from I(f, t). These units appeared to be located dominantly in the ventral parts of the auditory midbrain and had a latency exceeding 30 ms. On the basis of the action-potential wave forms the absence of non-separable I(f, t) in the incoming nerve fiber population is concluded. A spectro-temporal characterization of auditory neurons seems mandatory for investigations in and central from the auditory midbrain.