The aim of this study is to propose a bioindicator organism, the lizard Gallotia galloti, and a nondestructive biomarker assay, utilising serum butyrylcholinesterase, for the assessment of the toxicological impact of organophosphorus (OP) insecticides in the Canary Islands. Laboratory and field studies were performed using the OP insecticide Trichlorphon. In the laboratory study, experimental groups of Gallotia galloti were treated with 5, 50 and 100 mg/kg of Trichlorphon, respectively, and after 24 h the following enzyme activities were assayed: brain acetylcholinesterase (AChE), serum butyrylcholinesterase (BChE), microsomal carboxylesterase (CbE) and microsomal 7-ethoxyresorufin dealkylation (EROD). BChE activity was monitored in two groups of lizards treated with 50 and 100 mg/kg of Trichlorphon, respectively, for a period of 21 and 31 days after treatment. In the field study, BChE activity was detected in Gallotia galloti specimens, 24 and 48 h after treatment of an experimental area with 10 kg/ha of Dipterex sp80 (80% Trichlorphon). Three conclusions can be drawn. (1) Gallotia galloti has the features of an ideal bioindicator: high sensitivity to OPs and extremely slow recovery of serum BChE with respect to other vertebrate species; this property extends the temporal application of this biomarker in field studies. (2) A high correlation was found between the destructive biomarker brain AChE and the nondestructive biomarker serum BChE, 24 h after treatment. (3) The results of the field study show the relative 'non-toxicity' of Trichlorphon for nontarget organisms, such as lizards, at the average concentrations used in agriculture.