The team made flies with sperm that glows red and green
US researchers using genetically engineered fruit flies with glowing sperm have tracked the seed's progress inside the female, in real time.
By making the fly's sperm glow green or red, researchers from Syracuse University are able to see previously unobservable events unfold.
Competition between sperm is a key component of sexual selection.
The researchers hope their findings will also offer new insights into human reproductive biology.
'Knocked us out'
In nature, monogamy is often the exception, promiscuity usually the rule, the BBC's Matt McGrath reports.
But whenever a female of any species mates with more than one male there is a battle between the sperm of the potential fathers as they attempt to fertilise the eggs.
Scientists regard this type of sexual selection as a very important force for evolutionary change.
The trick of making fly sperm glow to see how they compete was first employed in 1999, by scientists from the University of Chicago.
Now the Syracuse University researchers have gone one better, by watching the sperm race in real time, the journal Science reports.
Prof Scott Pitnick says it was a jaw-dropping moment when he saw the multicoloured sperm through the microscope for the first time.
"It turns out that they [the sperm] are constantly on the move within the female's specialised sperm-storage organs and exhibit surprisingly complex behaviour," Prof Pitnick said.
"It far exceeds our expectations, in that we can essentially track the fate of every sperm the female receives.
"It's seeing all the novel observations, the complexity of what sperm do inside the female reproductive tract that no-one has ever been able to observe before. That's really knocked us out."
Prof Pitnick and his colleagues say they have created glowing sperm for other species.
They believe their technique could be used to understand not just issues of evolution but to potentially solve problems of fertility in humans.