A self-adaptive equalizer for use in a data modem for correcting in the baseband a received data signal. The equalizer comprises a difference circuit for determining the difference between a received signal and a correction signal formed at a sampling frequency by a transversal filter having adjustable coefficients, and which produces a corrected signal which is applied to a decision circuit which recovers the data signal. The output of the decision circuit is applied to the transversal filter, the coefficients of which are adjusted so as to minimize the mean-square value of an error signal. The error signal at each actual sampling instant is formed by deriving the algebraic difference between the value of the corrected signal at that sampling instant and the value thereof at a previous sampling instant, the latter value being first multiplied by the ratio between the value of the recovered data signal at the actual sampling instant and the value thereof at the previous sampling instant.