Field-collected Rocky Mountain wood ticks Dermacentor andersoni Stiles from British Columbia caused limited paralysis in hamsters at high doses, but not at lower doses, while ticks collected from Alberta failed to cause paralysis. This prompted development of a laboratory strain of ticks with a more consistent ability to cause paralysis. Progeny of ticks that paralyzed a cow near Walhachin, British Columbia were reared and assayed for paralyzing ability. A high-virulence colony was selected for four generations using female ticks that cause paralysis singly on hamsters. A low-virulence colony was concurrently selected using female ticks that failed to cause paralysis at four ticks per hamster. Logistic regression indicated that the paralyzing ability of the high-virulence colony was consistently higher than that of the low-virulence colony, but that the paralyzing ability of both colonies increased through four generations of selection. Ticks required storage at 10°C to terminate behavioral diapause. The effects of prolonged storage at 10°C on the paralyzing ability of ticks was estimated and was small compared with differences between colonies and across generations. Single ticks from the high-virulence colony caused an increasing proportion of paralysis with each generation, but only limited paralysis with no increase across generations for the low-virulence colony. Single ticks from the high-virulence colony that caused paralysis were similar in weight and percent weight gain compared with single ticks that failed to cause paralysis. Engorged weight and egg mass production after four generations of rearing in the laboratory were similar to that measured for field-collected adults. Results indicated that laboratory selection could increase paralysis, but not eliminate it. This was likely a result of an inability to select male ticks.