The computer program HYLAS generates from a standard DNA letter sequence a three-dimensional space curve (H curve) which embodies the entire information content of the original nucleotide sequence. The program can display H curves either as two-dimensional (front and side view) projections or as stereo-pair images. The curves can be marked at specific nucleotide locations, annotated, rotated for observation from any viewing angle, and manipulated for convenient side-by-side comparisons. Unlike the cumbersome letter sequences, H curves can be drastically condensed in size without losing their ability to reflect the global nucleotide-distribution pattern of the entire DNA sequence. Often, biologically important loci can be visually identified on the H curves. HYLAS is written in FORTRAN with separate mainframe (IBM-VM/CMS) and microcomputer (MS-DOS) versions. It uses the Tektronix-TCS library of graphic subroutines.