Three strains of Pseudomonasfluorescens (63-49, 63-28, and 15), one strain of Pseudomonas corrugata (13) and one strain of Serratia plymuthica (R1GC4) were tested on rockwool-grown cucumbers for their ability to reduce Pythium root-rot caused by Pythium aphanidermatum. These strains were previously selected for biocontrol ability from collections of >4000 bacteria. Strains 63-49 and 63-28 were tested on cucumber plants grown in rockwool in two replicated Pythium-inoculated trials conducted in British Columbia (B.C). Another inoculated, replicated trial was conducted in Quebec with all five strains. Cucumber yields (fruit number and weight) were measured over a ten-week harvest period. Strain 63-49 caused an early promotion of plant growth and increased cucumber yields at early harvests. No measurable effect of Pythium inoculation on disease development was observed in the Quebec trial, due to unfavourable cool weather. However, 63-49 significantly increased the total number of cucumbers (12%) and cucumber weight (18%), compared to the non-treated control. Strains 13, 15 and R1GC4 slightly increased the cumulative cucumber yields, but strain 63-28 had no effect. In the B.C. trial, inoculation with P. aphanidermatum reduced the number and weight of cucumbers by 27%. Treatments of Pythium-inoculated cucumbers with 63-49 significantly increased fruit number and weight by 18 %, compared to the Pythium-inoculated control. Strain 63-28 increased the cumulative number of cucumbers over time, compared to the Pythium-inoculated control, but the increase was less than with 63-49. The use of Pseudomonas spp. in rockwool-grown cucumbers can increase yields, both in the presence and absence of Pythium root rot, and with variable seasonal conditions and disease pressures.