We present observations of a rare , rapid , high amplitude Extreme Scattering Event toward the compact BL-Lac AO 0235+164 at 6.65 GHz .
The ESE cloud is compact ; we estimate its diameter between 0.09 and 0.9 AU , and is at a distance of less than 3.6 kpc .
Limits on the angular extent of the ESE cloud imply a minimum cloud electron density of \sim 4 \times 10 ^ { 3 } cm ^ { -3 } .
Based on the amplitude and timescale of the ESE observed here , we suggest that at least one of the transients reported by Bower et al .
[ 2007 ] may be attributed to ESEs .