Only certain DNA fragments are taken up efficiently by component Haemophilus cells; this implies that efficient uptake requires the presence of a specific nucleotide sequence on the incoming DNA (Sisco and Smith, 1979). To determine the structure of this "uptake site", we have isolated and sequenced four small fragments of cloned H. parainfluenzae DNA which retain the ability to be taken up by cells. These fragments have a sequence of eleven base pairs in common, 5'-AAGTGCGGTCA-3' and ethylation of certain phosphoryl groups in this sequence causes significant decreases in fragment uptake. We conclude that this is the sequence of the uptake site.