Light Emitting Diodes (LEDs) are an alternative to current greenhouse supplemental lighting technology due to their advertised durable construction, improved energy efficiency and selective spectral output. In addition, LEDs have the capacity to turn on and off high photon fluxes with rapid frequency (pulsed lighting). At the University of Arizona LED pulsed lighting was tested as supplemental light in greenhouse. Cucumber (Cucumis sativus 'Cumlaude') seedlings were grown until the second true leaf stage. Sunlight DLI (daily light integral, 7.6 ± 0.7 mol m-2 d-1) was supplemented with red-LEDs (661 nm) for 18 hours (2 am- 8 pm) with an average intensity of 60 μmol m-2 s-1 PPF. This provided an additional 3.89 mol m-2 d-1 of red supplemental light to DLI. The treatments consisted of (1) no supplemental lighting (control), (2) continuous red LED lighting, and (3) pulsed red-LED lighting at 50% duty ratio (2.5 kHz). The experiment was repeated twice. Results showed that both the continuous and the pulsed supplemental lighting improved the growth and morphology of cucumber seedlings, compared with those in the control. Significant differences between the continuous and the pulsed lighting treatments were observed for hypocotyl length. Plant hypocotyl length was greater in the pulsed lighting treatment than that of the continuous lighting treatment by 7.5%. However, there were no significant differences in number of leaves, fresh and dry mass, leaf area and chlorophyll concentration between the two treatments. We concluded that the pulsed lighting examined in this study (50% duty ratio, 2.5 kHz) could not substitute for continuous lighting in greenhouse cucumber transplants (cv. 'Cumlaude') as supplemental lighting since no benefits were observed and nursery propagators prefer compact cucumber transplants.