A series of pegylated distyryl boron dipyrromethenes have been prepared and characterized. Their in vitro photodynamic activities in Tween 80 emulsions have also been investigated against HT29 human colorectal carcinoma cells. The derivative having five triethylene glycol chains (compound 8) exhibits the highest photocytotoxicity with an IC50 as low as 7 nM. It is also localized preferentially in the endoplasmic reticulum of the cells and can induce predominately apoptosis upon illumination.