I'd first want to try and figure out why it's forming in the first place.
100% sure it's mold and not just dust collecting because of a lack of insulation over that area causing condensation?
Is it only on the outside walls?
Does the bathroom have a powered vent fan, and is it being used?
Is there a gap under the bathroom door so it can get make up air?
How much insulation is in the attic and what's your location?
Its a 1200 sft bungalow probaly built in 1960's There is no mold in bathroom but rest of house its around perimeters of ceilings. I cleaned areas and it doesnt seem to be growing back.Around foot of insulation in attic. Was wondering if there was a brand of paint to use(zinser?) Would hate to repaint ceilings and have problem coming back. House is located in Ontario Canada
Had aluminum soffit installed over wood. Contractor drilled extra holes into soffit area
That would seem to indicate the wall is getting cold and then warm,humid interior air is condensing on the surface in the winter. My guess is that you tend to notice the build up in the spring. Either no insulation in the walls, or perhaps blown in insulation in the walls that has settled. Johnyboy's suggestion is the easiest.