Commonwealth Games gold medallist Boyle had qualified fourth fastest with a 4:05.53 in the morning. While she was more than a second faster in the final, she found herself pipped in the tight battle for the podium with one second covering third to fifth placings.
Boyle won three bronze medals at the last worlds two years ago and was happy with the first of her three events in Kazan with her better events, the 1500m and 800m, to come.
''I was pleased with the swim to be honest, it was just a fraction outside my best at that distance," Boyle said.
"I think it sets me up quite well for my main events to come and I am looking forward to the 1500m freestyle tomorrow.
"A medal would have been nice for sure but the swim felt good to me which is the main thing."
Ledecky, a recent high school graduate who has yet to earn a driver's licence, was under world record time until the penultimate lap, but still had a hunch she had clipped that mark.
''When I touched I kind of thought that I had it," she said. ''But at the same time I couldn't be happier with another world championship gold medal. You really can't complain about that."
China's Sun Yang won the 400m for the third successive worlds in 3:42.58, taking to the podium with flashing lights on his shoes.
The rest of the opening day was a disappointment for New Zealand, with Matt Stanley in the 400m freestyle, Emma Robinson in the 400m freestyle and 100m breaststroker Glenn Snyders all off their best.
Stanley was nine seconds off his own national record in 3:56.79 and finished 37th fastest; Robinson swam 4:16.43 to be 6s slower than her best time; while Snyders clocked 1:00.44, .26s off his national record, but missed the semifinals by two places.
Boyle is back in the pool later today for the heats of her 1500m while Hawkes Bay 15-year-old Bobbi Gichard and American-based Corey Main are in the 100m backstroke, and Stanley returns for his favoured 200m freestyle.
