The present invention provides mutant microorganism that have higher lipid productivity than the wild type microorganisms from which they are derived while producing biomass at levels that are at least 45% of wild type biomass productivity under nitrogen replete conditions. Particular mutants produce at least 50% as much FAME lipid as wild type while producing at least the amount of biomass produced by wild type cells under nitrogen replete conditions. Also provided are methods of producing lipid using the mutant strains.