diff --git a/src/main.rs b/src/main.rs
index 87e26a0..38329b2 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -50,7 +50,11 @@ fn fallible_main() -> Result<(), NativeError> {
     info!("radicle-native-ci starts");
 
     let config = Config::load_via_env()?;
+    let mut logfile = config.open_log()?;
+    logfile.write("radicle-native-ci starts".into())?;
+
     let (run_id, run_dir) = mkdir_run(&config)?;
+    logfile.write(format!("created run directory {}", run_dir.display()))?;
 
     let src = run_dir.join("src");
     let log = run_dir.join("log");
@@ -229,6 +233,9 @@ struct Config {
     /// Directory where per-run directories are stored. Each run gets
     /// its own dedicated subdirectory.
     state: PathBuf,
+
+    /// File where native CI should write a log.
+    log: PathBuf,
 }
 
 impl Config {
@@ -241,6 +248,10 @@ impl Config {
         Ok(config)
     }
 
+    fn open_log(&self) -> Result<LogFile, NativeError> {
+        LogFile::open(&self.log)
+    }
+
     /// Read configuration specification from a file.
     fn read(filename: &Path) -> Result<Self, NativeError> {
         let file = std::fs::File::open(filename)
@@ -251,6 +262,31 @@ impl Config {
     }
 }
 
+struct LogFile {
+    filename: PathBuf,
+    file: std::fs::File,
+}
+
+impl LogFile {
+    fn open(filename: &Path) -> Result<Self, NativeError> {
+        let file = std::fs::OpenOptions::new()
+            .append(true)
+            .create(true)
+            .open(filename)
+            .map_err(|e| NativeError::OpenLogFile(filename.into(), e))?;
+        Ok(Self {
+            filename: filename.into(),
+            file,
+        })
+    }
+
+    fn write(&mut self, msg: String) -> Result<(), NativeError> {
+        self.file
+            .write_all(msg.as_bytes())
+            .map_err(|e| NativeError::WriteLogFile(self.filename.clone(), e))
+    }
+}
+
 /// How to run CI for this repository.
 #[derive(Debug, Deserialize)]
 #[serde(deny_unknown_fields)]
@@ -323,4 +359,10 @@ enum NativeError {
 
     #[error("failed to load Radicle profile")]
     LoadProfile(#[source] radicle::profile::Error),
+
+    #[error("failed to open log file {0}")]
+    OpenLogFile(PathBuf, #[source] std::io::Error),
+
+    #[error("failed to write to log file {0}")]
+    WriteLogFile(PathBuf, #[source] std::io::Error),
 }