I'm trying to remember what we did with head lice, our infestation was years ago. My best recollection is that the pesticidal shampoo wasn't really the key. The nit comb was. It took time, and patience, but I'm pretty sure that was shat really did the trick. I'll see if W remembers anything later, when she is home. Our lice came from (we think) the cute little caps our kids wore going to school in England. All the caps were stored together during the day, so if one kid had them, there was a really good chance a whole bunch of kids had them!
Edit to add: I'm pretty sure we used a conditioner first, to make it easier to use the nit comb on the kids' hair.
