We report the development of a line-scan focal modulation microscope (LSFMM) that is capable of high-speed image acquisition ( > 40 ?? fps ) with uncompromised optical sectioning capability. The improved background rejection and axial resolution of this imaging modality, enabled by focal modulation, are quantified with three-dimensional imaging data obtained from fluorescent beads. The signal-to-background ratio for the LSFMM system is one- to two-orders of magnitude higher than that for line-scanning confocal systems when imaging deep (up to 100 ?m) into a turbid medium of optical properties similar to biological tissues. The imaging performance of LSFMM, in terms of both spatial and temporal resolutions, is further demonstrated with in vivo imaging experiments with live zebrafish larvae.