The aim of the current study was to conduct usability testing of a mobile clinical decision support (CDS) prototype designed for urinary tract infection (UTI) assessment by nurses in nursing homes (NHs). Usability of the UTIDecide smartphone application (app) was evaluated using cognitive walk-through and think-aloud protocol sessions with nurses (n = 6) at two NH sites. This evaluation was followed by unsupervised field tests lasting ≥1 week with nurses at one site (n = 4) and posttest interviews and administration of the System Usability Scale (SUS). Cognitive walk-through/think-aloud sessions yielded interface design recommendations that were implemented prior to field tests. All test sessions resulted in highly positive perceived usability and usefulness from participants. Average SUS score was 92.5 (n = 3), which equates to an "A" grade for usability. Design recommendations identified for future app versions are: (a) integration of the mobile CDS app with organizational information systems; and (b) expanded features to support assessment of other conditions. [Journal of Gerontological Nursing, 45(7), 11-17.]. Copyright 2019, SLACK Incorporated.