Two '60s warm-weather favorites square off against each other in the first round of Ultimate Classic Rock's quest to find the Greatest Summer Song. You can vote for either the Beach Boys' "California Girls" or Sly and the Family Stone's "Hot Fun in the Summertime" below.

We’ve selected 32 of classic rock’s best summertime songs, and paired them off in 16 first-round duels. Over the next five weeks the field will be repeatedly cut in half by your votes, until we arrive at the greatest summer song ever made.

The Beach Boys have more great summer songs than any other artist, so coming up with just one to represent them wasn't easy. But "California Girls," recorded in 1965 a little before they made their masterpiece Pet Sounds, feels like summer wrapped up in less than three minutes -- from the opening carnival of swirling instruments to the glorious harmonies that propel it. Brian Wilson reportedly came up with the song during his first acid trip. It must have been one colorful and sun-kissed experience.

"Hot Fun in the Summertime," released in 1969 right around the time of Sly and the Family Stone's historic Woodstock performance, floats by like a lazy-day summer dream. It's sticky, it's sweaty and the sultry groove slowly drips like a melting popsicle. (Incidentally, the Beach Boys covered "Hot Fun in the Summertime" on their 1992 album Summer in Paradise, which is a long way from "California Girls.") Sly got deeper and weirder after this: Sunny "Summertime" eventually gave way to a darker autumn.

So which one will earn a trip to the second round? Listen to both below and make your call. You can vote once per hour in each of the 16 first-round Greatest Summer Song battles until voting closes on July 27 at 11:59PM ET. Your choice for the Greatest Summer Song of all time will be revealed on Aug. 25, 2015.

Listen to the Beach Boys' 'California Girls'

Listen to Sly and the Family Stone's 'Hot Fun in the Summertime'

See the Beach Boys and Other Rockers in the Top 100 Albums of the '60s