LOCUS is a database system for creating, manipulating, and interactively investigating databases. LOCUS was written in response to the need to study the data from the experimental devices at the Princeton Plasma Physics Laboratory and is now also used extensively at other fusion laboratories. An example of its use is the determination of scaling laws for tokamak plasmas. To do this, LOCUS allows the user to enter and modify data, define subsidiary variables as arbitrary functions of original data, plot and fit such variables, and perform multiple linear regression analysis. Through a flexible interactive interface the program is easy to learn and largely self‐documenting. There are two implementations of LOCUS: one that manipulates data in VAX/VMS ISAM files and another that manipulates data in the commercial relational database management system INGRES.