The rover/sitter polymorphism in Drosophila melanogaster larval behaviour is a unique example of a genetically determined, naturally occurring behavioural polymorphism. Allelic variation at the foraging locus (for) accounts for the rover (long foraging paths) and sitter (short foraging paths) phenotypes. We previously developed lethal tagging and used deficiency mapping to place for in the 24A3-C5 interval on the polytene chromosome map, thereby defining the for microregion. Here, we subjected this microregion to mutational analysis to (i) isolate putative lethal foraging mutations and characterize their behavioural phenotypes to assess whether or not for is a vital locus, (ii) generate cytologically detectable chromosome rearrangements with breakpoints in or near for for more precise localization and for future molecular analysis of the for gene, and (iii) identify other gene loci in the immediate vicinity of the for locus. We recovered 10 gamma-induced and 33 ethyl methanesulfonate (EMS) induced new mutations that define seven complementation groups in 24A3-D4. Two new EMS-induced lethal for alleles and four gamma-induced rearrangements with breakpoints in for were identified, which allowed us to further localize for to 24A3-5. All lethal mutations in for resulted in an altered behavioural phenotype providing evidence that both vital and behavioural functions are encoded by for.