When entering the name of a FITS table that is to be opened by a program, an optional row filter may be specified to select a subset of the rows in the table. A temporary new FITS file is created on the fly which contains only those rows for which the row filter expression evaluates to true. The primary array and any other extensions in the input file are also copied to the temporary file. (If a `#' character is appended to the name or number of the table HDU then only the primary array, and none of the other HDUs in the input file will be copied into the temporary file). The original FITS file is closed and the new virtual file is opened by the application program. The row filter expression is enclosed in square brackets following the file name and extension name (e.g., 'file.fits[events][GRADE==50]' selects only those rows where the GRADE column value equals 50). When dealing with tables where each row has an associated time and/or 2D spatial position, the row filter expression can also be used to select rows based on the times in a Good Time Intervals (GTI) extension, or on spatial position as given in a SAO-style region file.