In this paper we examine a technique to improve the performance of serial, matched-filter acquisition in directsequence spread-spectrum packet radio communications. Each packet transmission includes an acquisition preamble, and the preamble sequence is changed at the boundaries of predefined time epochs based on a pseudorandom sequence generator. The acquisition algorithm considered in this paper exploits knowledge of the preamble sequence employed during each time epoch to adaptively choose the acquisition threshold. The performance of the adaptive-threshold acquisition algorithm is evaluated and compared with one previously considered  in which the threshold is fixed. It is shown that the adaptive-threshold algorithm results in good average performance, even with a large a priori uncertainty in the signal-to-noise ratio. It is also shown that the average performance with the adaptive threshold is superior to that obtained with the fixed-threshold algorithm of .