The seed-feeding bug Lygaeus equestris changes and extends its food plant spectrum during its life-cycle. Whether this is the result of only the natural succession of the food plants, or also includes a change in the insect's food preference during its life, was the primary question of the present paper. Feeding-choice tests using ripe seeds from important food plants in the laboratory showed that Cynanchum vincetoxicum seeds were preferred during all stages of life. Among the lesser preferred seeds tested, a shift in preference occurred during the bug's development. It seems probable that this shift may be due as much to the physical as to the chemical properties of the seeds. It is suggested that most of the other plant species exploited for feeding by L. equestris probably serve mainly as substitute food when suitable developmental stages of C. vincetoxicum are absent or in short supply. Furthermore the paper gives some notes on duration of feeding upon seeds and effects of seed coat at initiation of feeding on Cynanchum and Helianthus seeds. During the third larval instar, the preference switches from the endosperm to the seed coat for initiation of feeding on Cynanchum seeds, and so it remains during the following stages. On Helianthus seeds, only adult L. equestris feed as willingly through the coat as directly from the endosperm.