Thraustochytrids, a group of fungus-like organisms belonging to the protist class Labyrinthulomycetes, are common colonisers and degraders of fallen leaves in mangroves and, thus, are actively involved in nutrient cycling. Mangroves, often located at the river mouth, constantly receive freshwater runoff, which contains organic and inorganic pollutants, including metals. Metals are known to cause cellular damage, which may affect the survival of thraustochytrids in the mangrove environment. A previous study suggested that Cu(II), one of the major metal ions in coastal water and sediment, retards the growth of and causes cellular damage to mangrove thraustochytrids. We hypothesize that increased concentrations of Cu(II) negatively affect the fecundity of mangrove thraustochytrids. In a laboratory study, we assessed the sporulation success (number of zoospores produced per colony) and the growth response (biomass) of 11 isolates of Schizochytrium limacinum collected from four mangrove stands in Taiwan exposed to increasing concentrations of Cu(II). Tolerance to Cu(II) varied among the tested isolates of S. limacinum . In general, a negative dose-response relationship was exhibited between growth response/sporulation success and increasing concentrations of Cu(II). However, exposure to low concentrations of Cu(II) had a stimulating effect on growth (2 mg l -1 ) and sporulation (2–64 mg l -1 ) for some isolates. A sharp decline in growth was observed at 32 mg l -1 Cu(II), and sporulation success was more tolerant to increasing concentrations of Cu(II). The IC 10 and IC 50 values for growth were 1.0–16.5 mg l -1 and 9.1–23.9 mg l -1 , respectively, whereas those for sporulation success were 0.6–40.7 mg l -1 and 10.5–108.4 mg l -1 , respectively. In conclusion, Cu(II) interfered with both the growth and sporulation success of S. limacinum , which may affect its abundance and distribution in mangrove environments.