This document presents a short guidance on the Threshold Approach for Acute Fish Toxicity. In the interest of animal welfare and efficient use of resources, it is important to avoid the unnecessary use of animals whenever possible. In the field of aquatic toxicology, this especially applies to the acute toxicity testing of fish according to OECD TG 203. The threshold approach described hereafter addresses fish toxicity by initially using a single-concentration test (limit test) requiring less fish compared to the full acute fish toxicity study. The selection of a single concentration is based on the derivation of a threshold concentration (TC) from reliable algae and acute invertebrate (e.g. daphnia) toxicity data. Fish toxicity is then tested at the TC to consider if fish are more or less sensitive than groups/species for which an E/LC50 is available. If no mortality occurs in the limit test using the TC, the TC might be used as a surrogate of the LC50 value in the further hazard or risk assessment.