Recurrent struvite crystalluria and urolithiasis in a cross-Labrador bitch was studied using a combined Coulter-Counter and scanning electron microscope (SEM) approach. Staphylococcus bacteria were cultured from the patient's urine as well as from the calculi themselves. Urine samples were subjected to particle counting and sizing during active and non-active periods of stone formation. Size distribution curves so obtained were identical as were those derived from sterile and non-sterile specimens. These showed a peak incidence at a diameter of 5 microns. Particle sizes for 6 controls were also determined and showed an even distribution over a much wider range with small peaks occurring at 3, 10, and 20 microns diameters. SEM studies of urine sediments revealed the presence of struvite crystals in all the controls as well as in the stone-former. These occurred in a variety of shapes and sizes but were generally larger in the controls. SEM also revealed intimate admixtures of struvite and apatite in calculi surgically removed from the patient. The results of this study indicate that crystal numbers are of greater significance than crystal size. It is also suggested that Cross-Labradors may be unusually predisposed to struvite crystalluria. The repeated recurrence of struvite urolithiasis in the subject indicates a possible inherent physiological malfunction in the animal's ability to cope with this crystalluria. The absence of a nucleation inhibitor in the stone-former's urine is also postulated.