We examine the hypothesis that population variability is created and regulated in the juvenile stage for demersal marine fish. Juvenile mortality is examined for 17 populations of Atlantic cod (Gadus morhua), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), plaice (Pleuronectes platessa), yellowtail flounder (Pleuronectes ferrugineus), and sole (Solea vulgaris) in the North Sea, Irish Sea, Barents Sea, Baltic Sea, and Northwest Atlantic. A latent variable covariance analysis of recruitment time series with measurement error is used to test hypotheses. We found strong evidence of density-dependent mortality within cohorts during the juvenile stage for cod, plaice, sole, and whiting; density-dependent mortality appears to be related to the log of juvenile abundance. There is evidence of negative autocorrelation between adjacent cohorts of cod; this pattern is consistent with density-dependent mortality between adjacent cohorts. The autocorrelations are positive for the flatfish examined. It is possible to obtain estimates of estimation error variances for populations if there are multiple research surveys of the same population. We conclude that the juvenile stage is very important for population regulation in most species but that the source of the variability in year class strength is in the larval stage or very early juvenile stage.