ABSTRACT: Growth production of the euphausiid Nyctiphanessimplex Hansen was examined off the southwest coast of the peninsula of Baja California, México (Bahía Magdalena, 24 to 25* N), as a function of seasonal changes of temperature, zooplanktonbiomass, upwelling index, and regional ocean circulation. The data were collected during 4 oceanographic surveys from June to November 1986. High densities of N. simplex were found in the shallow coastal waters (&LT300 m). High production ismaintained during late spring and summer, decreasing during autumn when stratification of the water column develops. We estimate the production attributable to growth of N. simplex on the coastal shelf was 273.42 mgm^-2yr^-1 and showed a strong seasonal variation. This value was about 1/4 the value previously reported at Bahía Vizcaíno in northern Baja California (28 to 29* N). Our data supports the hypothesis that there are strong latitudinalchanges in regional production of this species along the west coast of Baja California. The production:biomass (P:B) ratio obtained for the period June to November 1986 was 6.99 yr^-1. The maximum reproductive activity of N.simplex during spring and early summer at Bahía Magdalena appeared to be related to enhancement of the coastal upwelling.