Cigarette butts are the most common form of litter worldwide, as an estimated 4.5 trillion cigarette butts are deposited somewhere into the environment every year. With cigarette consumption on the rise globally, along with the increasing popularity of bans on indoor smoking, the global environmental burden of cigarette waste may increase in the years to come. Many chemical products are used during the course of growing tobacco and manufacturing cigarettes, the residues of which may be found in cigarettes consumed and therefore in the butts discarded. These include pesticides, herbicides, insecticides, fungicides, and rodenticides. Additionally, over 4,000 chemicals may also be introduced to the ambient environment via combusted cigarette particulate matter (tar) and mainstream smoke. These include chemicals such as carbon monoxide, hydrogen cyanide, nitrogen oxides, polycyclic aromatic hydrocarbons, ammonia, acetaldehyde, formaldehyde, benzene, phenol, argon, pyridines, and acetone, over fifty of which are known to be carcinogenic to humans. Furthermore, chemicals such as arsenic, nicotine, polycyclic aromatic hydrocarbons, and heavy metals have been found to leach into the environment from cigarette butts littered along roadsides and in laboratory studies Using standard acute fish bioassays, cigarette butts were analyzed for aquatic toxicity. The LC50 for leachate from smoked cigarette butts (with remnant tobacco intact) was approximately 1.1 cigarette butts/L for both the marine topsmelt (Atherinops affinis) and the freshwater fathead minnow (Pimephales promelas). Leachate from smoked cigarette filters (remnant tobacco removed), was less toxic, with LC50 values of 4.1 and 5.5 cigarette butts/L, respectively for both fish species. Lastly, unsmoked cigarette filters (no tobacco) were also found to be toxic, with LC50 values of 5.1 and 13.5 cigarette butts/L, respectively for both fish species. Consequently, toxicity of cigarette butt leachate was found to increase with smoking the cigarette, and again with leaving remnant burnt tobacco intact. Additionally, the marine topsmelt was found to be more sensitive to most cigarette butt leachates than was the freshwater fathead minnow. This study represents the first in the literature to investigate the toxicity of cigarette butts to fish, and will assist in assessing the potential ecological risks of cigarette butts to the aquatic environment.