The loss of parasitism in metazoan lineages is often seen as unlikely, but it has occurred in some lineages (e.g., leeches, lampreys). How and why parasitism is lost is aptly addressed by studying lampreys, because extant species include a range of feeding modes and parasitism has been lost repeatedly. An individual‐based model was developed to determine if variations in survival and growth rates in the larval and juvenile stages could favor parasitic or non‐parasitic strategies. A realization of the model for a Lampetra spp. population, a genus which includes parasitic and non‐parasitic animals, indicated that both strategies could be successful. A different model realization of the non‐parasitic species Lethenteron appendix also agreed with expectations, only non‐parasitic strategies were successful. Modeling anadromous Petromyzon marinus produced only parasitic animals, as expected, but suggested two different adult sizes should appear in the population, which has not been reported in the literature. Finally, a realization of an Ichthyomyzon castaneus population, only known to produce parasites, rarely selected for parasitism (~7% of model iterations), possibly because the population used to parameterize the model was unusual for the species. The results suggest that non‐parasitism in lampreys is common because parasitism, while offering better growth, also has lower survival. Additionally, non‐parasitic species may be generated at different rates because growth and survival thresholds in the model favoring parasitism are close to observed estimates in some populations. Loss of parasitism can occur when species have different tradeoffs in growth environments between life stages.