In a study on the nature of induced genetic variations in quantitative characters using the selfpollinated plant Arabidopsis thaliana, it was observed that not only quantitative performance but also intraline variability, or developmental instability, was affected by induced mutations. Emphasis was placed on the latter.Seeds of Landsberg strain were used. Two irradiated (20 kR and 80 kR) populations together with a control population were propagated by self-fertilization for six generations using the "one-parent to one-offspring" scheme. In M3 and in M6 generations, plants were taken at random and their M4 and M7 progeny lines were grown to study the possible occurrence of polygenic mutations. Both in M4 and M7 lines, in addition to an increase in genetic variances, the environmental component of variation was also increased. The mean and within-line standard deviations were found to be uncorrelated in M7. A selection experiment showed the differences in developmental instability among M7 lines to be genetic. From these it was concluded that radiation has induced a genetic change causing an increase in intra-line variability, or developmental instability.Selected higher and lower instability lines were grown under different environmental conditions. It was observed that with constant temperature, the developmental instability increased, but with changing temperature it decreased. Each environment was assigned a value based on the mean performance of all lines in this environment. Phenotypic plasticity of a line was measured by the regression of this line on environmental value. It was found that phenotypic plasticity was not correlated with developmental instability.