The acute mortality (14-day LC50) to the guppy (Poecilia reticulata) of reactive organic halides was determined. These compounds were significantly more toxic than the non-reactive organic chemicals with unspecific toxicity. The 14-day LC50 of 15 compounds was correlated with hydrophobicity (P oct ) and a chemical reactivity parameter (reaction rate constants with 4-nitrobenzylpyridine). The quality of the QSAR with chemical reactivity as parameter was much better than of a QSAR with P oct