Dike growth is analyzed, on the basis of the fluid-dynamical behavior of incompressible, Newtonian magma in a spatially uniform fissure of temporally variable width. The length and width of the dike vary with the magma pressure at the dike entrance connected to the magma chamber and with the mean stress working normal to the dike walls. The mean stress consists of the regional crustal stress and the mean elastic stress, which is approximately evaluated using an analytic solution of the crack with a constant aperture. The length and width of the dike are determined as a function of time by simple ordinary differential equations. The dike can grow when the initial pressure of the entering magma is above the lithostatic pressure and high enough to suppress compression of the crustal stress. The assumed size of the initial crack affects the growth only in the duration needed for an initial adjustment stage. After this stage the width and length first grow exponentially, and then the growth slows as magma is released from the chamber. The growth in the later stage is classified into two types. When the initial magma pressure is high enough, the dike elongates unlimitedly while the width finally decreases. This extrusive growth is likely to lead to eruptions. The other intrusive growth, in which the dike finally stops its growth with a finite length and width, occurs when the crustal stress component normal to the fissure is of sufficiently strong extension. Buoyancy of magma is not very effective in the earlier growth stage, but it shifts the marginal condition separating the extrusive and intrusive growths. The extrusive and intrusive events that recently happened in Izu-Oshima volcano and Izu-Tobu volcano group, Japan, are interpreted as the examples of these two growth types. ¿ 1999 American Geophysical Union