A filter-testing fixture holds a single fuel filter and is adjustable to accommodate a range of filter sizes and shapes. The fixture has a base section that defines a water collection reservoir, and the base opens upward toward a housing. Water separated from fuel in the housing thus drains into the reservoir. A fuel inlet conduit passes through the reservoir, one end of the conduit being disposed above the base section. The first, or filter-supporting, adapter is removably attached to this end of the conduit, and both the first adapter and this end are disposed within, the housing. The housing removably and sealingly mates to the base, and a cap assembly likewise removably and sealingly mates to the housing. A second, repositionable, filter-holding adapter is connected to the inner surface of the cap.