A circuit for use in the add-compare-select circuit of a Viterbi decoder to determine the maximum likelihood path through states in a coding trellis uses a modified butterfly structure. Selection registers select between branch metric values at time i and partial path metric values at time i-1 in dependence on comparisons of branch metric difference values at time i and partial path metric difference values at time i-1. The selected values are added to provide new partial path metric at time i. The circuit provides a significant reduction in power consumption and area over conventional designs.