A petition by the Center for Biological Diversity to have the Hermes copper butterfly listed as an endangered species was denied by the Obama administration.
Ilene Anderson with the Center for Biological Diversity said without Endangered Species Act protection, one of Southern California's rarest butterflies could disappear forever.
"It's a great disappointment because this is a species that is very threatened by its extinction because of some of the big wildfires that we've had in San Diego as well as development pressures," said Anderson.
Anderson said the butterfly was instead placed on the growing list of "candidate" species despite threats to the yellow-orange, spotted butterfly.
She said the U.S. Fish and Wildlife Service said the butterfly warrants federal protection but won't get it because protection is "precluded" by actions to list other species.
Anderson said the "warranted by precluded" designation doesn't provide protections for species and many have gone extinct while waiting.
She said the Hermes copper butterfly only exists in San Diego County.
Conservation groups have sought protection for the threatened butterfly for almost 20 years. Petitions were first filed with the federal government in 1991 and again in 2004.