"I am quoting the official data from today's temperature measurement: 'Navalny A.A., strong cough, temperature 38.1'," he wrote, referring to degrees Celsius, or 100.6 degrees Fahrenheit.
"P.S. I am continuing my hunger strike, of course," Navalny said.
The opposition politician is serving a two-and-a-half year sentence on old fraud charges in a penal colony some 100 kilometers (62 miles) east of Moscow known for its harsh discipline.
His latest statement from the colony comes after pro-Kremlin media on Friday launched an offensive aiming to disprove his complaints of mistreatment and lack of medical attention.
Two reports in pro-Kremlin outlets described Navalny as looking "quite normal" and saying he is incarcerated in a colony that is "practically exemplary."
In his post Monday, Navalny said the reports had "not a single word of truth."
As evidence, he wrote that a third person out of 15 total in his unit had been hospitalized with tuberculosis since his arrival at the penal colony in February.
"I am surprised that there is no Ebola virus here," he quipped, adding: "such is our 'ideal, exemplary colony'."
Navalny was arrested on his return to Russia in January, after spending months in Germany recovering from a poisoning last summer that he blames on the Kremlin.
Earlier this month, Navalny, who is considered a flight risk by authorities, filed two complaints against prison officials, saying he is woken eight times a night by guards announcing to a recording camera that he is still in his cell.
