A morphological and morphometrical study was made of the tobacco cyst nematode complex, Globodera tabacum sspp. tabacum (GTT), virginiae (GTV), and solanacearum (GTS) including observations of eggs, second-stage juveniles (J2), males, females, and cysts. Observations focused on the anterior region including head shape, lip pattern, stylet morphology, and the tail region including tail shape in J2 and spicules in males. The head region of J2 was set off from the body and consisted of three head annules, six lips, and an oral disk. The shape of the head seen in lateral view showed little variability within or among the subspecies. Stylets of J2 were robust and had three anchor-shaped, rounded knobs. They varied slightly in width and height within each subspecies. The lip region consisted of a central oral disc surrounded by two lateral and two submedial lips. The oral disc varied from rectangular to elliptical. The submedial lip pairs were not fused in all specimens. The tail of J2 was finely pointed and the tip was rounded in all subspecies. The head of the male was set off and contained five head annules, six lips, and an oral disc. The head region was similar in lateral view in all three subspecies. The stylet was robust and had three rounded knobs that slope posteriorly. The dorsal knob of males of GTV appeared to slope more posteriorly than in the other two subspecies. In the SEM, the head region had a large central rounded to elliptical oral disc with two lateral and two submedial lip pairs. The submedial lips were rounded or rectangular. The submedial lip pairs were fused in most specimens, but not all. The spicules of GTT showed a slightly more enlarged head region than the other two subspecies. The three subspecies could not be separated on the basis of any character or group of characters of J2 or males.The anterior region including body shape, head shape, lip pattern, stylet morphology, and the terminal area in females; and body shape and terminal area of cysts were observed. The most useful characters to separate the three subspecies were female body shape, stylet knobs, perineal tubercles, cyst shape, anal-fenestral ridge pattern, anus, and tail region. GTT was characterized by rounded females and cysts with sharply back sloped stylet knobs, clumped perineal tubercles in the vulval region, tight parallel ridges in the anal-fenestral region of the cyst, and a distinct tail region not shaped like a crescent. GTV was characterized by its ovoid to ellipsoid female and cyst shape, the "Dutch shoe" shape of the dorsal stylet knob, the more individualized perineal tubercles, a maze-like pattern of ridges in the anal-fenestral region, and an indistinct anus. GTS was characterized by its ovoid to ellipsoid female and cyst shape, moderately backward sloped stylet knobs, more widely separated ridges, a distinct anus, and a tail region usually shaped like a crescent. Much variability in shape and patterns was visible among all the isolates of the different subspecies. Tubercles in the neck as well as bullae were reported and discussed. A morphometric evaluation of second-stage juveniles (J2), males, females, and cysts was performed for several characters. Morphometrics of eggs, J2, and males were considerably less variable than in females and cysts. No measurements of eggs and J2 were useful for identification of the three subspecies. The distance of the median bulb to the head end and the distance of the excretory pore to the head end in J2 and males were quite reliable and useful. The stylet knob width of males was useful for identifying GTV isolates. Tail length was useful in separating males of GTT isolates from GTV and GTS. The body length/ width (L/W) ratio of females and cysts discriminated GTT from GTV and GTS. Stylet knob width of females was an auxiliary character useful for identifying GTV. This subspecies complex has a continuum of values for the majority of the characters observed. The data suggests a closer relationship between GTV and GTS which is likely because these two subspecies occur in very close proximity in Virginia, 38 km separation between closest adjacent geographical ranges; whereas GTT occurs 700 km away in the northeastern U.S. / Ph. D.