A query to a nucleotide database is a DNA sequence. Answers are similar sequences, that is, sequences with a high-quality local alignment. Existing techniques for finding answers use exhaustive search, but it is likely that, with increasing database size, these algorithms will become prohibitively expensive. We have developed a partitioned search approach, in which local alignment string matching techniques are used in tandem with an index. We show that fixedlength substrings, or intervals, are a suitable basis for indexing in conjunction with local alignment on likely answers. By use of suitable compression techniques the index size is held to an acceptable level, and queries can be evaluated several times more quickly than with exhaustive search techniques.