Citrus solvents are made from the extract of citrus peels, and have an ingredient called d-limonene that kills all stages of the flea. Given that NY State has voted to take flea bombs off the market for consumer use because they are considered so toxic, and that the bombs are being reviewed for their toxicity elsewhere, finding a safer method of getting rid of fleas is a godsend for many. Just note that cats are sensitive to d-limonene, and that it is a volatile organic chemical (if a natural one), and can aggravate asthma.

1/4 cup citrus solvent
1/4 cup liquid soap or detergent
2 gallons warm water

Combine all the ingredients in a bucket. Stir to blend. Mop the floor; rinse.