Abstract: Five populations of Trogoderma versicolor Creutz. were established in Bulgaria from1998 till 2006: in different wheat varieties in storage at Plant Protection Institute (PPI), whichwas bread under laboratory conditions 6 years long; in storage with packed wheat germs inconfectionary factory in the town of Burgas; in wheat grain storage in Draganovo village; instorage with herbs in the town of Shumen and in storage with cocoons of Bombix mori L. inSVILA factory in the town of Haskovo. The resistance to phosphine in adult insects and 4th-5thinstar larvae in these populations was investigated. The experiments were conducted in thelaboratory for stored product insects at Plant Protection Institute. It was found that the highestresistance to phosphine showed the population from Haskovo. The LD50 values for the adults andthe larvae were 0.0358mg/l and 0.1180mg/l, respectively; the LD99.9 values 0.1350mg/l and1.8810 mg/l, respectively. Resistance showed also the population from Draganovo – the LD50values for the adults did not differ from those in the laboratory population (PPI) (0.0058mg/l and0.0041mg/l, respectively), while according the LD99.9 values their resistance was visible. Theresistance in the larvae in this population was good expressed – the LD50 and LD99.9 values were0.0487mg/l and 1.0780mg/l, respectively. Susceptible to phosphine were the laboratorypopulation and the populations from Burgas and Shumen. The larvae from all tested populationsshowed higher resistance to phosphine compared to the adult insects. The increased resistance tophosphine in the populations led to a longer larval development (68-97 days), lower weight of thefemale pupae (2,93-6,43mg) and lower reproductive potential (the number of laid eggs variedfrom 10 to 86) compared to the same parameters in the susceptible populations. The populationsof T. versicolor from storages and foodstuffs which have been treated many times with phosphinewithout obtaining sufficient efficacy showed increased resistance.