The rock band from San Jose performed during a live concert in South Dakota Sunday as part of the 80th annual Sturgis Motorcycle Rally.
Video from KOTA-TV of the concert shows a crowd gathering near the stage with little social distancing and most people not wearing masks.
Singer Steve Harwell could be heard addressing the pandemic and using expletives to describe COVID-19.
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One person tweeted, "Ugh I am so disappointed in them. They were suppose to be better than this."
"He really ain't the sharpest tool in the shed," tweeted another person.
"Man, Smashmouth used to be one of my favorite bands back in the 90s! Saw them live a few times. But would I risk a pandemic to see them 20 years later? Hell no!," wrote one person.
More than 250,000 people are expected to attend the 10-day Sturgis Motorcycle Rally, which began Aug.7 and will run through the 16.
Upcoming feature concerts include Buckcherry, Lit, .38 Special, Quiet Riot, Reverend Horton Heat and Bone Thugs-n-Harmony.
Masks are encouraged, but not required.