A waste product of SynCrude Canada Ltd.'s oilsands mine is mature fine tailings (MFT), a toxic aqueous suspension of particles, organic acids, bitumen and metals. One disposal method involves constructing lakes lined with MFT and capped with clean water. Prototype ponds support fathead minnows (Pimephales promelas; FHM), but the long-term viability of these populations is unknown. This study attempted to determine if exposure to MFT and tailings pond water (TPW), a related waste product, affected growth and reproduction of FHM. Laboratory larval growth bioassays (7 and 56 days) yielded no significant differences, but larval fish spawned in MFT water and TPW grew faster than control larvae. In a life cycle bioassay, male FHM raised in TPW showed delayed development of secondary sexual characteristics and were less likely to spawn. In the prototype ponds, no patterns in body size and condition were evident, but spawning was delayed in one MFT site.