Interbrood period, gonad development, and total lipid content throughout the oogenesis and spermatogenesis processes of the subtropical euphausiid Nyctiphanes simplex were studied. Specimens were collected during six oceanographic cruises in Bahía Magdalena (March, July, and December 2004) and in the Gulf of California (November 2005 and January and July 2007). Females attained first spawning when ∼7.5 mm total length (>52 days old). Histological evidence indicates that N. simplex females have group-synchronous ovaries, able to produce four broods per gonadic cycle, since ovigerous females develop simultaneously in three and four distinct substages (Oc1, Oc2, Oc3, and Oc4) in their gonads. Once females mature, as shown by pale pink gonads, they may reabsorb their gonads in <4 days. Direct observations indicate that after a variable resting period, the formation of oogonia to vitellogenesis takes ∼3 days, investing ∼8% (4-14%) of weight-specific carbon body weight to reproduction (lipid approach) with an average interbrood period of 10 days (range: 7-26 days, estimated by three distinct methods). About 22% of the ovigerous females in the metanauplius stage show gonad development in vitellogenesis, likely spawning between 7 and 9 days. The rest of the female population have an interbrood period that is considerably >10 days. Embryonic development in the ovigerous sac last <3 days (16 °C), hatching always as nauplius (usually 100% hatching success); the metanauplii are released from the ovigerous sac in a median of 5 days after spawning. Although sac-spawning euphausiid species may have comparatively lower total fecundity than broadcast-spawning species, they seem to have relatively similar reproductive effort and higher hatching success that increases larval recruitment rates, compared to similar size temperate broadcast-spawners. This partially explains why sac-spawners of the genera Nyctiphanes, Nematoscelis, and Pseudeuphausia are numerically dominant euphausiids in several highly eutrophic temperate, subtropical, and tropical ecosystems. N. simplex males have a continuous spermatogenesis after they attain size at first maturity; continuously allocating ∼5.4% of weight-specific carbon to reproduction, results that are significantly different than previous assumptions that euphausiid male spermatogenesis is energetically insignificant. © 2009 Elsevier Ltd. All rights reserved.