Abstract Experiments were performed to examine the effects of spoilage and ensilation on the thiamine-destructive properties of whole sprats, Sprattus sprattus L.. The thiaminolytic factor was shown to be heat-labile, with pH optima of 4.5 and 6.4. This confirmed that the factor was an enzyme, thiaminase. Thiaminase activity was found to persist during spoilage and immediately after ensilation, but was significantly reduced by extended storage of silage at ambient temperature. The effect of mixing thiaminolytic silage into a moist pellet with a dry binder meal was investigated. This was shown to suspend thiamine destruction, suggesting that thiaminase-containing silage may be used safely to produce moist feeds for fish, given that an adequate supplement of thiamine is included in the binder meal.